Not counting Vaati and Ganon, the already established recurring villains, I'd definitely like to see more of Majora, and for a number of reasons. The first is plain and simple: I loved the boss fight in Majora's Mask. It was challenging (the first time through, without the Fierce Diety Mask of course) and was overall an interesting battle and conclusion to the game. Second, I'd like to learn more about the origins of the mask and how the sentient spirit inside came to be. What the Happy Mask Salesman said in MM really seemed like a TL;DR version of what may have actually happened. My third and strongest reason for wanting to see Majora's return is the inevitable revisit to Termina. After all, it ranks high on my list of favorite overworlds and had a lot of great things going for it. Seeing a new take on it would be a real treat, and I do think it's about time that we return to Termina.
I'd love to see Vaati again. He was such an interesting villain to me, and deserves more recognition. If he could reappear in the child timeline, what's to say he can't reappear in one of the other timelines? Probably not the adult timeline, since it's flooded, but he could very easily reappear in the Downfall timeline. The Palace of the Four Sword hasn't been so much as mentioned in this timeline, but that doesn't mean it doesn't still exist. After all, it wasn't mentioned in OOT, MM, or TP, but then randomly reappeared in FSA. I think there's plenty of room to bring back Vaati. Maybe we'll get a 3D Four Swords game on that timeline or something (I doubt it, but a guy can dream, right?)
I would also like to see Demise again. Throughout the entire series, he has had ~15 minutes of screentime (could be less, could be more, depending on the player), which is a shame because he was such a cool villain! I actually discussed in an earlier thread the possibilities of Demise appearing in A Link Between Worlds. I am beginning to have my doubts, put the possibility is still there (though I think the Yuga/Hilda combo sort of disproves this). If he doesn't appear in this game, then I would love to see him in Zelda Wii U. He was a very underdeveloped and rather vague character, so I would love to get more explanation. He could, theoretically, appear in all three timelines, given that each one contain the sacred blade that he is sealed within.
The Nightmares! They scared me so much when I was young(er) even more so when I learned that they were all working against Link. That and they can take many different forums. Granted they couldn't really return unless we have another dream game, but even if a new boss one shows up which has the powers of the nightmare, THAT would be one interesting game!
I had to go with vaati! I know its only a few games back, but TP was the last time we faced ganon on a console...right? that was a good few years back! So i would like Ganon to return on a console game! However i would like Vaati to make his way into a Console game. For some reason the GC version of Four Swords can hardly be counted as an actual LoZ adventure, sorry just how i feel!
Vaati was cool, and I would like to see him in a more Important Role. Dark Link is a given for anyone. Some want the Mystery of Dark Link to stay as it is; In Question! Some want to see more of him and learn about him. I am a Dark Link Hybrid, I dont care what they do with Dark Link.... Just use him! I Love the whole Doppelganger approach with characters, Like Nega Duck in Darkwing Duck.... Love that guy! Proto-man/Break-man from Mega Man, Cloud and Sephiroth.... Kinda...i guess...Final Fantasy is just weird! but you get my drift!
Definitely Dark link, back when AoL came out games were a less story oriented, I would love the whole concept of Dark link expanded on, especially considering he's the opposite of Link, the opposite of courage, of good.
My vote is obvious, Dark Link all the way! Oh and Dark Link can talk if want's him to. Besides if the Hero's Shade talked, so can Dark Link. And another thing is if Dark Link is bad guy version of Link, he would have motives. Dark Link's motives would be opposite to Link's, instead of bringing prosperity, Dark Link would bring terror as he wreaks havoc on the world and most of all ruin Link's life. If you ask me, that sounds like a "main villain".
Honestly? If we're talking about reusing final boss battles, then none. The final boss battles have been great usually, but that doesn't mean I want to see them again. If we're talking about just the actual villains themselves, then I guess I'd like to see the obligatory Ganondorf again.