What we know:
-Tyreal is fighting with the spear
-Imperius is fighting with the spear
-Diablo is fighting Imperius
-Tyrael vs Imperius happens
-Tyrael's tabard is dark when he fights imperius, and bright white when he's holding the spear
-Diablo has multiple heads
-All the angels/demons are originally from the same being
-Diablo is the final boss
What is suspected:
-Tyrael versus Imperius happens before the game, resulting in Tyrael being the fallen star
-Imperius is the 2nd to last boss
My theory on how it all goes down.....
-Diablo escapes the void by absorbing the other primes, and possibly some of the lesser evils becoming a "super diablo", explaining the multiple heads. Possibly has something to do with the black soulstone, but maybe not (I think it's more likely the black soulstone will be one of the lesser evils attempts to do what diablo is doing, in a race against diablo)
-Super Diablo invades heaven, beats Imperious down, and somehow enslaves him (only way I can see Imperius both fighting Diablo, AND being a boss later on...)
-Upon beating Imperius, he monologues a cheesy redemption speach, tells tyrael he was wrong, warns him about how strong diablo is, and then Merges with Tyrael to try and make a celestial being strong enough to counter the super diablo (Explains Tyrael's color scheme shift, his holding the spear)
Still we dont see Tyriel using El'druin while firghting Imperius, so we can asume he dont have it cause it is destroyed or something, so the fight between them didnt happent before game :) Still i dont wanna think Tyriel is bad guy, but i belibve Imperius pwned diablo , then Tyriel merged with Diablo :P and he is final boss....Still we see that Tyriel destroy something with Solarion, it must be Diamond gate entrance.
And we forgot something , we are main cars in game, we will kill last boss not Tyriel, so i dont belive Tyriel merged with Ipmerius just to kill diablo cause we are the ones who will do it. Your teory is pretty intersting but i dont belive that killing diablo is done by super being but only by us. I can see we are killing super being, and that being is Tyriel/diablo merge. And i dont think Tyriel will do it cause he become evil, but cause he think he can contain Diablos esence within him for a greater good. If u look closely u can see that Tyriel is destroying diamon gate entrance, why would he do it is diablo is already inside , and if diablo already killed Imperius.
But again i think your teory is right cause i cant see way how could tyriel kill imperius.