Now that you mention it, that sounds like the most likely outcome. I don't think he'll release Calamity Ganon since he is Calamity Ganon, or at least, the source of it, but he does make the most sense from a lore perspective, and it would be odd to see Yuga show up since Lowrule doesn't seem to be a thing at the moment. Vaati is the second most likely outcome though, because iirc the whole unleashing Ganon thing is his shtick.
It could be possible that this is Vaati, and Ganondorf will be shown off later.