Originally Posted by
Veluren
I imagine of the three it will probably be Galakrond.
Chromatus never had a huge impact on WoW itself, since he was only ever in a book and I don't think he's ever been brought up in the game proper.
Murozond we know the circumstances of his death, so if he's the "final" boss of the expansion it would end with him escaping so we can kill him in the End Times later (earlier?), which wouldn't feel good to not get to bump off an expansion's main baddie (side-eyes Garrosh).
Galakrond is something that's known of in-game, talked about a fair deal, and is a thematic existential threat to everything, not just the dragons, so his return, however they do it, would set him up best as the final boss.