I think it would have to depend on the timeline. In general, however, as much as I like Demise, I think he needs to wait a while. If Ganondorf is the villain, he should be revealed right from the start, otherwise we'll have to go through the same debates we did with Twilight Princess. I for one fail to grasp the appeal of Vaati, but if it's in the Adult timeline, sure, why not? Put an overrated villain in a timeline that started with an overrated game, and maybe you'll have something I'd actually be able to follow the hype for. However, I'd rather see Vaati as the villain for A Link Between Worlds, so I'm ruling him out. For one thing, I think the Downfall Timeline is pretty much stuck with being on the handhelds, so that leaves the Child and Adult timelines.
Personally, I would love to see Veran look like this
in an actual game. However, I think that if you have Veran, you kind of need to have Onox as well, and that path leads to a retread of the Oracles game, although if they do it right, that could be a good thing.
In terms of the DS games, Malladus is much more interesting than Bellum, who kind of just comes across as that timeline's version of Nightmare. Malladus does have a backstory, so the Wii U game could be a kind of prequel to Spirit Tracks.
Ghirahim could come back without having to make his plans be about bring back Demise, but it's really unlikely. Granted, it would be cool to see him having calmed down since the events of Skyward Sword.
So yeah, I want an old villain to come back, just so we can develop them, rather than bringing in another new villain.