Of the nine main villains, five didn't reform: Queen Chrysalis, King Sombra, Lord Tirek, the Dazzlings, and Principal Cinch. Of those, Cinch is the only one who is neither dead nor has a somewhat ambiguous fate.
Of the other four, we all have our headcanons on what happened to the Dazzlings after losing their power, whether Tirek is damned, living imprisoned in Tartarus, or was eaten by Cerberus, and whatever we think happened to Chrysalis after her fall.
All this is not taking the comics into account where Sombra is reformed, Nightmare Moon is dead, and Chrysalis is full karma houdini.
Bringing any of them back will kill our headcanons, and retcon what was supposed to be for the one I didn't mention. There is a good chance that if any are brought back they'll reform just like Discord and Starlight when they brought them back (though Starlight was obviously planned from the beginning of the season).
So, if you had to choose one of them to bring back and possibly reform, who would it be?
Here's my order, from most to least:
1: The Dazzlings
They are my first choice hands down. They're a bit like Discord - they cause disharmony and are fun to have around, though in a different way. Yes, every other time a human character loses their magical source of immortality after a hundred years or more, they age rapidly and die. This is called No Immortal Inertia. However, if MLP is willing to make an exception to that rule, I'd like to see the Dazzlings return for redemption. Though we're not entirely sure if they have any return value since they can no longer sing, and we loved them for their songs, I kinda have a soft spot for Aria Blaze. (blush) Also Sonata almost reminds me of Pinkie Pie.
2: Lord Tirek
I'd really rather him stay in Tartarus after what he did to Twilight. And since Tartarus is hell, I'd like to think that that Dragonball Z style fight ended in the climactic Dragonball Z way. (besides, we see Frieza and Cell in DBZ hell, which is called HFIL (Home for Infinite Losers).) Having him return would, of course, retcon my "damned souls are in cages" theory.
Besides, if he is alive he's powerless, and the cage would then be there to keep him from ever getting out. Even if somepony broke him out it'd be impossible for him to be a threat again. And if he did, either a.) The Mane 6 can use Rainbow Power to send him back, b.) Discord would send him back (I do not think he'd betray the ponies for Tirek again), or c.) The Mane 6 can't use Rainbow Power again, Tirek takes Discord's magic, and Equestria is defenseless against him (which of course would never happen in the show).
3: Queen Chrysalis
Even though what Tirek did to Twilight was more permanent, what Chrysalis did to Twilight was more drawn-out and made me feel more depressed. Even though she is my favorite villain, she brought about the single most depressing moment in the show by turning all of Twilight's friends and Celestia on her without even putting them under a spell (Shining Armor I think was, though), for what could have been a simple misunderstanding, and if it had been, what would have happened to Twilight? Would she have been imprisoned in Tartarus like Tirek? Probably banished and thrown into a dungeon in the place she was banished to. Of course, what would have happened if Chrysalis hadn't imprisoned Twilight herself is irrelevant, because Chrysalis would have waited longer and become powerful enough to kill Celestia, and taken over Equestria herself. Also, if Cadance hadn't proven to Twilight who she was, Twilight would have killed her and died of starvation in the caves.
So, for this reason, I want her dead, and my interpretation of her fate is that she fell to her doom. If they did decide to bring her back, I'd be open to it only if they handled her return and defeat the right way! (No karma houdini or temporary imprisonment, and... well, if you want to know the rest, read this blog.)
Edit: Actually, I forgot that if she does, a wise and respectful Chrysalis lover has given me a headcanon to go by.
4: King Sombra
Above all else, I'd prefer if they didn't have him return outside of flashbacks, alternate timelines, dreams, hallucinations, etc. Chrysalis could have survived her fall and be living in exile. Tirek could have been teleported to Tartarus with a cage conjured around him out of thin air. The Dazzlings could still exist as regular teenage girls. Sombra's fate on the other hoof is more definite. He was shattered to pieces in a spectacular flash of light. He was pronounced deceased by the episode's writer. He proved that this show isn't afraid to do what G1 did twice, and what many other kid-friendly shows have done, often in more brutal ways. For me this was the most powerful moment of the show until the Cutie Mark Crusaders got their cutie marks, so if they said he survived and retconned the one official death of a main villain this show has, it'd be like taking away the CMC's cutie marks to me. (yes, I know some of you are probably tired of hearing this)
Besides, to Chrysalis and Sombra we already got alternate futures with them in power.
Now for the poll: (say your order below too if you want)