So... As Heaven is now divided (theory) with Imperious being possibly corrupted by Diablo and makes it very clear he despises Nehalem. Also Archangel Malthael has gone mad and been replaced by Tyreal. This combined with the crystal arch being compromised and previous destruction of the world stone and the concept of the Black Soul stone. Trang'Oul would not be impressed with this at all.. Trang'Oul would want to restore balance to Sanctuary and thus make his presence known to all who would stand in his way or Aid him.
The Necromancer has been a servant of Trang'Oul as we have learned from Diablo lore he would be likely an important character to either bring back to the fold or at least include as a more formidable NPC.
Perhaps Trang'Oul will just want to wipe the slate clean and instead of being a good thing for Sanctuary he would be a bad thing.. Perhaps Tyreal with his Wisdom realizes that Trang'Oul is going to re-balance the world and the heavens and the hells all at once. This would be a "unjust" move in his mind requiring the return of the aspect of Justice! However wisdom alone is not enough as his powers have been stripped when he became a mortal... Tyreal would need to ascend in order to reclaim the aspect of Justice..
Archangel Malthael would need to be either found and reminded of his role or be destroyed... Imperious would also need to be dealt with in some fashion. Tyreal in his quest to become an Angel once again would no doubt need a little Nephalem help.
But Hell is not forgotten in all of this... Since Heaven is divided and the Prime evil is destroyed something must take its place... Imperious (now corrupted) is the only being powerful enough to claim this duty... Perhaps Trang'Oul will sense this and want to stop it or maybe he will endorse this move as it fills in a role necessary for balance?