Oh, and for the record, what does Tl;Dr mean?....
Too Long; Didn't Read. Something you would use especially for Genmotty.
When you're joking, do try to add

or whatever. Even then it can look a bit insulting, which that does. I know it's a joke, but would, for example, Genmotty take it as a joke if he saw it?
Anyway, I feel that at the moment, the Tau have no nemesis. To give the Tau an arch-enemy, you need to give someone the ability to truly hurt them. Someone to give them a sharp kick that makes them fight for their lives.
Oh, sure, losing hundreds of thousands in Tyranid or Ork attacks is tragic. Such a shame. But that was for the greater good. Orks and Tyranids are like animals. It wasn't an act of evil. It wasn't a deliberate assault.
And while we're at it, what are the Tau, as a people? What would drive them to hatred? Well, they're not communists, no matter what the jokes say. Not truly, at least. The Empire is a dictatorship. The Empire is led by what is in effect personality cults based around the ruling caste. Alpha Wolf's joke and Comic Book Guy's speculation for was IMO spot on to reality. Oh, sure, it might not fit what they claim, but here's the thing, people bought what Stalin sold, or at least pretended to. Assume that's happening with the Greater Good and the Empire really is falling for it. The Tau are truly quite Stalinist, and they claim to be something they are not.
And that brings us to the analogies.
What provoked Stalinism to hatred? Invasion. How did Stalinism respond to the invasion beforehand? Ignorance, arrogance, and disbelief.
Even if we were to assume that the Tau's propaganda is true and they're a melting pot society of equals as they claim, this still draws two real world analogies instead. An economically powerful, relatively young state, with the means to build up massive military might if so provoked.
Once again, like the United States in 1941 and 2001, the Tau would likely be blind to the threat, and that would be the main spur for the anger. The numbers don't matter in such things, it's principle. It's whoever launches a deliberate assault.
Only one race has ever done that.
The Damocles Crusade wasn't much, and the Tau seem to have largely ignored it. But what about the next time? One species has the precedent and the motive. Eldar don't really care enough, Dark Eldar, ditto. Chaos has bigger fish to fry, and besides, a human is a human to the Tau. Orks, Tyranids, already dismissed. Necrons, well, probably just a myth to the Tau as well. They're not a Tau nemesis, they're Daleks, they hate everything.
No, there is only one choice for a nemesis for the Tau. The Imperium of Man.
All they need is someone to give them the cause for the Tau to realise it.