I'm pretty sure the demon Garona heard was just the fel version of Medivh. The question is, is the fel version of Medivh just a corrupted Medivh, or is it something more? I think his demon form was more or less a corrupted version of himself (like Gul'dan's spikes), but that doesn't mean he wasn't acting as someone's avatar in that form (since he wasn't remembering it anyway). I'd guess that the existence of the Burning Legion as the true masters of the fel will be a major revelation in a future movie (at least for the characters).
On that note, I'd be willing to guess that next movie ends with Gul'dan losing support of the Horde and fleeing to his masters, similar to the events of our Second War. I don't think he'll be torn apart at the Tomb of Sargeras though, but instead turned into the Lich King. Ner'zhul was not in the movie, and from what I understand, not in the prequel novel either. I think his role will be rolled into Gul'dan's, since they're honestly pretty similar. Despite Ner'zhul meaning well, he was the first to sell the Horde out, and got turned evil right after trying to redeem himself anyway. It'd be a lot simpler to have that just be Gul'dan, who also sold his people out and was evil all along. Also, they made a big deal about the fel always having a price, but no one really paid yet (Medivh succumbed to it, but he wasn't trying to use it). However, I'd expect his death to be relatively vague, depending on whether or not they want to fully reveal his masters yet, and his rebirth as the Lich King to not be revealed until later.
EDIT: Come to think of it, Medivh's demon form reminds me a lot of this old art of "The Deceiver".
https://hydra-media.cursecdn.com/wow...everMetzen.jpg