Pick one hero at Marvel or DC that you'd like to see become a bad guy. Pick one bad guy at Marvel or DC that you'd like to see become a hero.

Fine, I'll start.

Hero to villain (Marvel): Wolverine. Not in a short-term mind-controlled way, but fully committing to being a bad guy. He's Vandal Savage crossed with the Punisher, and could be a pretty serious threat.

Tony Stark.

Granted, Tony Stark as a villain is kinda sorta Lex Luthor, but Stark would have much more class. And hair.

I really like the idea of Wolverine as a villain. It is actually surprising that he isn't. As far as X-Men go, Kitty Pryde would be a hell of a good cat burglar. Hell, Cat Burglar would be her nom d'crime!

Bad to good?

Doc Ock could be a Reed Richards type easily enough.

I'm tempted to say Doc Doom, except, from a certain perspective, he's already there. It's really just his oversized ego that's keeping him from being Marvel's greatest hero...

I would like for someone other than Carol Ferrris gain control of the Star Sapphire and become a major DC heroine.

The Wasp (along with reviving her of course)
The Green Arrow

Moon Knight comes to the DC Universe, and instead of being his usual mixed up multiple personality good guy, he becomes the new Anti Batman.

Red Tornado goes to Marvel Comics and becomes the Vision's alter-ego, and evil to boot.

These are very cool. Would like to read them.

Reading GL at the moment?

Hero to Villain:

DC - Allan Scott
Marvel - Captain America (Steve Rogers)

For Scott, the catalyst could be losing Jade, though I guess it may echo too close to Parallax. Would be interesting to see, particularly considering how he has ties to such a large section of the DCU.
For Cap, just seeing how others would respond to him. And I'm not talking Civil War disagreement, I'm talking flat out villiany.

Villain to Hero:
Marvel: The Lizard.
DC: Black Manta

Marvel Hero-to-Villain: Vision (easily done with reprogramming or a virus or something)
DC Hero-to-Villain: Green Arrow (I sort of already consider him a villain anyway lol)

Marvel Villain-to-Hero: The Crusader (let's have a good Christian for a change instead of a zealot)
DC Villain-to-Hero: Rampage

H2V- Scarlet Witch
V2H- The Scorpion

Hank Pym to villain
Mad Thinker to hero

Well, IIRC he was shown admitting that maybe he had gone too far, so perhaps he's due for a Damascus Road moment.

I'd vote for Mysterio to become a hero. I always thought his use of gadgetry was cool. And he's a Spidey villain. They've been known to go straight now and then.

Wolverine would be a great choice for villain-to-hero. I have a hard time considering anybody that bloodthirsty a hero in the first place.

At DC, the original Crime Doctor was a sympathetic bad guy. He turned to crime partly for the thrill and partly to ensure through good planning that nobody got hurt during his heists--and he gave his cut to charity. He could see the light and become a real hero.

Hero to villain--let's see, what about Kyle Rayner? He could go insane, nearly destroy the universe, and then...no, on second thought that would probably create more trouble with fans than it would be worth....

On the DC side of things, it'd be kind of cool if the Barry Allen that returned became a villain, being the Reverse-Flash to Wally's Flash.

Oh my, that WOULD be asking for trouble from fans!

Oh, I know. I just think it'd be interesting for the Reverse-Flash to unquestionably be smarter and more experienced than the Flash, not to mention the personal connection.

Good to bad:

Hmmmm. Brainiac 5 would make a good enemy for the LoSH.

Namor is arguably better (more interesting) as a bad guy.

Bad to good:

Icicle from the Injustice Society (as a member of JSA)

Molecule Man (with Volcana)

One of my favorite examples of a villain-turned-hero (or at the very least, anti-hero) was Doom 2009...

Would be neat, but may echo a bit too closely to Kyle Rayner and Hal as Parallax.

True, in a way. I kind of think it'd work better, though, since Wally's already established as The Flash, and has been for years, and there's an actual relationship between Wally and Barry.