As a note, the hint for the Tau was that "We've intercepted alarming signals from the Damocles sector! For the greater good!"
Which seems to me like the time period that it would be meant to take place in would be the Damocles Crusade. In which case, the Kor'Or"Vesh would have been in development, with the Kor'Vattra making up the majority of Tau fleets. And to be honest, if we can have both, I'll be glad for it. Kor'Vattra vessels tend to be a bit bigger and tougher while still being cheaper than Kor'Or'Vesh, even if they're also slower and weaker in boarding and firepower.
I'm guessing for torpedoes though, what they'll do is that for every torpedo an equivalent Imperial ship would have, Tau ships will fire three/six homing missiles that each deal around 12/6 damage. It balances out the strain of additional entities the game has to keep track of and display, as well as visual aesthetics, with the Tau lore of networked missiles. Theoretically, they could have each of the three appear like a cluster of three missiles, but it would introduce problems and visual effects when destroyed and everything that it would probably just be better to have three or so. The reduced total damage mostly compensates for the fact that they're homing (as well as meaning that Tau have yet more reasons to hold back), though if they took off armor-piercing and left it at 15 damage per missile that could accomplish something similar. (Actually, thinking about it, the fact that Tau have to identify the targets before they can fire probably compensates for the increased accuracy at that point, so 15 damage per missile is probably fine) Probably like 90° forward launch bays in that they'll need enemy ships to be identified, and could either have a random chance to detonate after certain distances, or limited turning rate so that if they're dodged they get exposed to turret fire again, but otherwise unlimited distance like normal torpedoes.
Manta Heavy Bombers would probably be fairly simple changes. When launched, it's a squadron of 1 Manta and two/three(playtesting would have to see which is more appropriate) Barracuda Fighters. Mantas get 30% base chance to dodge hits, can take three hits before they actually go down, and if only two Barracudas are launched alongside them, would probably be able to shoot down ordnance like a Fighter as well. Deal 30/45 damage a hit, with 7.5/11.25 critical chances and attack 3/2 times. They hit at full strength until they go down, which is the primary reason testing would have to be done to see if three Barracudas makes them too hard to fend off for balance.
Prow Deflectors are easy to implement, just have them act like an additional Prow Weapon or die out when the Generator gets hit. If they wanted to, they could do something similar to the Eldar, where the engines influence the shields, since Tau use their gravity sheaths as their primary means of travel in the void, as well as shields. If Engines get hit, it could cut the shield regen rate in half, but only gives half as large of a speed penalty. Same would go for Generators. Give 75 armor in the front while active, of course.
As for accuracy, my guess/idea would be that it's the same as Imperial accuracy, however, Messengers and Custodians get Advanced Tracking Systems or Advanced Targeting Relays, whatever they want to call them for this version. Any allied ships with Tau Accuracy (As opposed to Imperial or Auxiliary) within X units of the ship with the system, including the ship itself, get a 20% bonus to accuracy. This also applies to turrets, though maybe they would cut the bonus in half. While this boosts accuracy at all distances, not just past 6k, it means there's still a purpose for the Tau in the 'accuracy penalties reduced beyond 6k' upgrade, and is more useful at long distances. The radius of effect is up for debate; I suspect they'd just standardize it at around 3k, as it's easier and doesn't make too much of a difference. Messenger will already be quite vulnerable in this game as compared to TT, so giving it a bit more leeway to hang back isn't too bad, and Custodian should be fine with 3k range. We'll see, of course; they could also just give it as an activated skill or only work when Lock On is activated, as well. Ah, and 10,000 Detection range for ships with the systems as well. They could also have it so the bonus given to allies is lost if the Deck is destroyed, or have it be like a Weapon System, or have it completely tied to the Deck and the bonus be lost in it's entirety if the Deck is destroyed. *shrug*
It would be nice to see Auxiliary vessels, but I don't know how well they'd fit into it. They could be done with their own unique set of skills or favours, or cost additional renown to start with and act as pre-favoured vessels...it would certainly be interesting to see how the Demiurg ships would be handled, though. I suspect many of the upgrades will be flavored after auxiliary technology.
I think I saw someone mention that Tau wouldn't be able to get lost in the Warp; while technically true, the drives that power their Warp dives can malfunction, and have limited power supply, so they could as easily be stranded in realspace for long enough to fill the same role as the Warp.
On a similar note, they'll probably be Loyal, as while Tau aren't as independent as Imperial officers, they're more willing to retreat as part of combat doctrine, and may find the decision to fall back to be the more intelligent one, even if that doesn't agree with their Kor'O. Hopefully mutinies would be more about the on-board (low-ranking) Ethereal(who's rather inexperienced with actually commanding the vessel...) taking charge, or the crew telling their captain that he needs to step down if he is unwilling to give for the Greater Good. Commissar equivalent would obviously be Ethereal. Execute becomes Override, perhaps?
Though on the subject of crew members, I want to say what I figure the rest would be even if it's unnecessary. Navigator might remain Navigator though with a picture of either a Nicassar, navigational computer, or Technician(not sure which Caste it'd be). Techpriest becomes Earth Caste, Ratings become Fire Warrior Teams, Servitors become Drones, and Gunnery Master and Squadron Sergeant, of course, remain the same.
And Favours...Tindalos really has four routes they can go here, I think. They can by Sept with some Castes thrown in, or Auxiliaries with some Castes thrown in, just Castes, or all three. Castes are basically certain to play into it, it's just too much a part of the Tau not to tie in. I'm rather hoping they go for all three, with Demiurg/Earth Caste/Bork'an or Dal'yth, Kroot/Fire Caste/Vior'la or Sa'cea, Nicassar/Air Caste/Vash'ya or Ke'lshan, and possibly Human(would far rather they just be called human than Gue'la, if we won't be using the Tau name for the Kroot or other auxiliaries), Vespid, or Nagi for the last, as those are the best known other auxiliaries, though they each have issues with being tied in as a favour here. Could also just be Ethereal and a focus back on Tau. Probably no Water Caste, as they're involved anyway with their being alien species involved, and also just don't have much to do with the fighting of the war itself, even if they're involved in all the support and diplomatic deals associated with the war.
I figure the actual bonuses might be something like this.
Fire Caste/Kroot Favour:
Kroot Hunting Packs: +10 Troop Rating, +1 to assault actions when Boarding.
? Crisis Assault Teams: Allows use of Manta Dropships, which launch two Assault Actions if they reach an enemy vessel.
? (For either ships without hangar bays or if assault boat-equivalents are found to be unfitting even with the melee favour, I really have no idea what else the active skill would be)
Earth Caste/Demiurg Favour:
Field Testing: +1 Upgrade Slots to the Ship/+1 Skill Slot to the Ship.
? Nanotech Repairs: The ship will regain 2(3?) Hull Points per second for 45 seconds. All fires on board the ship are extinguished. One critically damaged component is repaired. (One weapon system is repaired?) Cooldown: 180 seconds.
? Automated Repairs/Expert Technicians: The cooldown of Emergency Repairs is reduced by 25%.
? Psychic Detection: The designated enemy ship is identified. Cooldown: 120 seconds
? Multidirectional Gravitic Sheath: The ship boosts in the designated direction without needing to change it's facing. Cooldown: 60/120 seconds. Speed Factor: 60/80?
? Veteran Crew(?): Every crew member gains one free crew point.
? Superior Gravitic Sheath: The Consumption Gauge refill rate is increased by 25%
The Greater Good/The Aun Bids Us Fight: This ship is immune to insubordination.
? Inspirational: Reduces the cooldowns of all nearby ships OR boosts fire rate and reduces cooldowns of the ship for a short time. (Probably not) Cooldown: 180 seconds
? You Will Come To See Our Ways (?): Forces an insubordination check on the targeted ship. On a failure, the ship automatically mutinies, but does not attempt to flee. Cooldown: 180 seconds Range: 6,000 OR equivalent of Slaanesh Favour active skill
No idea what they'd do for Humans or Vespid. They could also have a stealth and/or damage boosting skill, called Kauyon or Mont'ka (although I will take care to remind those of us who may have forgotten that Kauyon and Mont'ka are Fire Caste strategies based off of their own history as hunters. The Kor do not have the same background as the Shas. While they may certainly be influenced in their strategic thinking by the most militaristic caste, they will have their own doctrines, although they may be similar in some aspects.
Hopefully I'm not repeating too much of the OP, by this point. Half-trying to do a compilation of what's been discussed, so...
Weapons would probably fit somewhere between Chaos and Eldar. Either multiple turrets with 180° turning ranges, a few with 270°, High RoF, and few attacks per volley per turret, or several larger turrets with same ranges, still decent RoF, and higher attacks per volley. Range can be converted as usual, and Macro DPS should be somewhere around Chaos-Imperial, though, especially as Tau rely on their cheap escorts. Speaking of which, with Gravitic Hooks, Orca/Warden escorts could benefit from the upgrades and passive favour bonuses of their parent ships, if that was how Tindalos wanted to represent it. They could just ignore the Gravitic Hooks entirely, which I'm hoping they don't do, as the escorts seemed like a big part of Tau strategy in the TT. It was where most of their Lance power came from, short of Demiurg allies, from what I could tell, for instance.
Shields and Hull Points can be converted directly from what's in the TT same as they have everything else, though I believe it would be better if they used the tougher variant of the Merchant, and of course the Defender and Castellan would be Heavy Escorts. Kor'Vattra vessels obviously have lower troop values, and reduced number of Boarding Actions, and no lightning strikes for any Tau ships, of course.
I can think of a few Upgrade names off the top of my head, such as Demiurg Ion Cannons for the range boost, Ramming Wedge for a boost to ramming damage and an upgrade to Prow Deflectors to allow them to work against Ramming/Asteroids/Explosions as well, though the ramming boost is lost too if the Deflectors go.
Playstyle would probably have two main types; Skirmish/Artillery and Alpha-Assisted/Mid-Range Brawl (it's not really a brawl at that point, I know, but Tau will be dishing out enough Macro fire at that level that it's rather similar). Focus on either the excellent Ordnance and bomb the Warp out of the enemy, or focus on the excellent long-range Macro fire where you can let your Prow Deflectors soak up damage as you wear out the enemy, all of whom currently lack a good amount of forward-facing long-range firepower. Eldar crumble under either one if they can't get behind you and cause you to crumple with your weaker, Assault-poor ships only as tough as they are. Chaos will lose the Ordnance fight if they don't get in close and use their better brawl capability (without taking enough damage to lose it...), though Lance fleets against long-range macro Tau will be...interesting, and probably depend on who brought more ordnance. Orks just need to get up close, and will probably smash through any long-range macro that doesn't flank them really carefully. Imperials are the closest to Tau, with enough Ordnance to not get utterly smashed by it like Orks, matching speeds to prevent truly effective kiting, and heavier prow armor to blunt the macro fire as they close, although the Tau won't be bothered by the torpedo volleys. For Ordnance fleets, it'll be the usual, though macro fleets will probably come down to whether the Tau can prevent the Imperials from breaking their line, and keep the Brawl at Prow's length, preferably out of AP macro/Lightning Strike range.
Sidenote: All Tau ships have gravitic sheaths if they're larger than a Manta. They just can't be focused as Deflector wedges in smaller vessels, and smaller vessels can't mount the rather oversized FTL drives the Tau have to use.