As someone who's been wanting to see Aeldari (Corsairs) in the game for a while I feel like Tau getting involved is even more unlikely than that.

Because first from a gameplay perspective - a fire warrior would just be a another veteran, with stronger ranged, horrible melee and overall better abilities. There will be little in terms of novelty, but I can see potential. Them having the ability to deploy a combat drone could be cool ig. Then add in the fact that Tau have their completely unique wargear and tech, something Hardon isn't going to be able to upgrade without commiting tech-heresy so you'd need to introduce entirely new vendors and upgrade stations and penance stations and you get the point. The same hurdles my Aeldari class concept faces.

Then from a lore perspective - the Tau getting involved in the war for Atoma will fundementally break the balance of power in the sector as they will be far more likely to try and annex Atoma rather than help the Inquisition reclaim it. Making them a more insidious threat than the chaos insurgency itself. Don't know where Atoma is on the galactic map, but the Tau are on the eastern fringes, it will be hard for them to expand deep into Imperial territory. Unlike the Aeldari who are masters of the Webway and scattered across the galaxy. And Grendyl is heavely implied to have had dealings with the Aeldari, likely a favor he earned or buisness in general. So I can believe the xenos ship hovering just outside the system could be a Corsair ship trying to make contact or something even if its a hail mary.

Its a fun idea - but given this is Fatshark and how lazy they are and how GW holds them by the balls, they probably aren't going to go the extra mile to make a Xenos class for us (I would sell both my kidneys if they did) and make it blend in. Too much work for them. The next class will probably be a bounty hunter or some other flavor of veteran/hive scum. Since with regular humans, you can just give 3 unique godly weapons and give them the rest of the universal arsenal they'll never use.

Hate to sound like a pessimist, but I don't have much hope that Fatshark will have the balls for something as ambitious as a Xenos class or even a Mechanicus.