This would actually be a great way to help both Thrall and Jaina's characters grow if it was Thrall that destroyed Theramore. There are two ways I could see that going down:

1) We already know Garrosh is going to become corrupted. So say Garrosh tricks Thrall. He begs Thrall to come help him, there is some convincing reason he gives that Theramore needs to be tsunami'd right now, or the world will be in grave danger. Thrall is forced to choose between saving the greater good or saving a small human town. He chooses the former. Thrall and Jaina both must come to terms with this choice.

2) There actually IS a reason to tsunami Theramore. I don't know what it could be-- but it would have to be big, with far-reaching consequences, and the rest plays out as above.

A third way that still involves Thrall but less directly is say there is a tsunami, or something, approaching Theramore. Jaina pleads with Thrall to come help, she can't do it on her own, and the draenei and dwarf shaman are not powerful enough. Thrall is torn between some Big Important Event with World Consequences (as above) or saving Theramore from this natural disaster. Again, coming to terms, blah blah blah.

It would be so good for both characters and the story as a whole if it was Thrall who did this to Jaina, but the writers would have to come up with a good reason for Thrall to do this (tricked into thinking it was for the greater good, or it actually being such) because Thrall is not a dick, as much as some players think he is. ;P