Cyborg Superman for sure. I think he developed a lot as a character during his Sinestro Corps run and I'd be sad to see him disappear.
Darkseid, obviously (I know he is returning...)
Neron is good. He's got a lot of untapped potential.
Mordru, obviously. And by that I mean outside of his Demon Knights run.
I'd like Sinestro to return to being a bad guy. IMO, he's been brilliantly written recently.
Extant/Monarch
I'll think of some more later. This obviously doesn't include standard rogue galleries.
I don't want to see Doomsday. As far as I'm concerned he should have stopped at H/P. He hasn't been the same since then.
I don't want to see Amazo because he's hax and the idea of someone creating an android that can do what he does is stupid.