Sorry Kot I'm too sleep to read your post just now, so I'm just going to post my reply to Rolepgeek's nice wall. Sorry if I say something you countered, I'll read it tomorrow X_X nevermind I read it, pretty cool. I think maybe the chaos gods always existed, paradoxically before they were created by humanity and Slaanesh by the Eldar? meh
I'm just posting for funsies, I really like reading theories and coming up with them (: I'm still not even especially well versed in the lore.
Huh, glad I didn't write up a big reply because Rolepgeek said what I was going to, but a lot better O_o And then I wrote a bunch anyway
Though allying with the orks seems like a stretch maybe... But eh, the Tau do work well with the kroot, and it would be more "Fight *those* guys, they're TOO STRONG!" than "Let us hug and be allies~".
Yeah though... Tau see the effects of chaos. They can't be psychers, but they interact with plenty of human psychers. Probably even gue'vesa. There's nothing stopping them from taking the human psycher equipment which protects against the warp, and making it better. Humanity can't do that because, idk, scientists fall prey to Tzeentch I guess. Not a problem for Tau.
Science is their greatest strength. They've advanced an incredible amount in such a relatively short time, too - they may literally be experiencing a technological singularity. They're certainly advancing orders of magnitude faster than humanity did, even with the Emperor. The Eldar could be helping them yeah, but... I don't think that fully explains their tech.
I'd bet a lot of their occupied human colonies have formed chaos cults, too. Especially with the Tau promoting science and open mindedness. So the Tau have opportunities to study those cults, learn how dangerous they are, and how to stop them. They're experts at social engineering and "harmony", whatever their methods.
I don't know much about the Eye of Terror, but it does seem like the Imperium is sitting on a dead-man's bomb there. But it's not like the Eldar, Tau, or even Orks want chaos to break free. *Especially* the Eldar- Slaanesh is one of the chaos gods, after all. So even if the Imperium is too stubborn to explain the Eye of Terror to the Tau, the Eldar would out of simple self-preservation. And the Tau reaction would be "Oh, guess we'll stop absorbing the Imperium then for now" because they're actually willing to *think* and cooperate, things the Imperium is nearly incapable of.
Then it's just a race between the main hive fleet, the necrons reawakening (and rounding up the c'tan shards I guess), and the Tau.
The Tau don't have any disadvantage against tyranids, in fact their naval technology has the potential to far outstrip the stagnated Imperium. So if any faction has a chance, it's the Tau (or chaos, but chaos is bad).
Pretty much ditto for the Necrons, except they'd have help from the Eldar- who literally exist to fight Necrons, and have all the necessary intel to make it possible.
And I could really see the Orks helping out against any end-times threat, just because they want to keep fightin. Especially against the biggest, baddest enemy. They'll never be unified enough to wipe out the Tau, even if they wanted to, they're just a spoiler.
(Emperor turning into a full god is also possible, but I think it's hard to speculate about how that would even happen. I'm not sure what he would even do- crusade against chaos and then all xenos, like before? I guess probably. Seems like something the Eldar would know how to prevent, since they're practically experienced at god-creation now.)