Since that wont happen and this isn't GW2 sadly, its more likely blizzard has a plan in mind, my own two cents however in a way that makes sense by blizzards cliches is that Garrosh either:
1. Is sent into exile and consequentially gets finished off by Vol'jin as the Troll promised.
2. Is sent into exile, only to be taken in by the very creatures he fought against, the burning legion, vowing revenge on all of Azeroth soon enough.
The latter is possible because of the fact we know there is an "Iron Horde" template for the next expansion, even if it was only a texture model its actually part of Garrosh flag which implies that we haven't seen the last of Hellscream.
Again, the idea of a reoccurring antagonist to me is a good idea, it gives us a familiar face to love to hate and long to finish off once and for all.