As we all know, villains can become playable or a team-up in Marvel Heroes IF they have been on the side of the angels at least once, thanks to that yin-yang magic mumbo-jumbo or otherwise. So far we have only gotten those that have been popular enough within the community to garner some attention to the developers, such as Juggernaut and Doctor Doom, with the exception of Ultron, who nobody saw coming and Loki, who released in conjunction with Thor: The Dark World (why it wasn't Lady Sif, we may never know).
But what of the villains that aren't as popular? The villains of old that come back up attempting some kind of redemption (usually guided by a hero) or the villains that have attempted some kind of heroism in one way or another? I may not have listed every kind below, but only because the likes of Grizzly and Porcupine probably wouldn't make substantial playables (I would definitely have Grizzly follow my Ant-Man along, though).
Lizard
Curt Conners has never willingly become a villain, but he did willingly become the Lizard. Normally a lethal opponent to Spider-Man, there were times when Conners had control over the Lizard and used its abilities to assist Spider-Man, including when Morbius the living Vampire bit him, transferring Conner's persona into the Lizard body (don't ask me how that works). Although always ending up back as the mindless creature again in the end, the Lizard could be a possible candidate for playable hero.
Prowler
A rather D-list character almost verging on C, Prowler started off as a criminal dealing petty crimes across New York (window washing wasn't cutting it for him). He never did feel inclined to hurt people, so that's a plus, and his criminal career was short-lived, so his validity on even being on this list are slim, making him just as much a "villain" as Hawkeye or Wolverine. Nevertheless, his reputation as a hero flies under the radar for most (along with his existence altogether) and is known mainly for his old feud with your friendly neighborhood. If his powerset doesn't cut it as playable hero, then a Prowler team-up would do just fine.
Morbius
I'll bet you're starting to see a pattern of Spidey adversaries. Michael Morbius' first appearance was as he was traveling aboard a cruiser to the United States, where he drank the blood of the entire crew and attempted to commit suicide. Fun times. Unlike Prowler, Morbius stayed an antagonist for some time, coming to blows with Spider-Man, the Human Torch and the X-Men. After this he went very much on his separate path with his fiance, Martine Bancroft, taking care of evil-doers such as Daemond, the Living Eraser from Dimension Z and a Zombie Virus. Morbius has done a fair bit of hero work and is very much worthy of becoming a playable hero.
Songbird
Have a breather from the Spidey villains while we move on to Melissa Gold, originally known as Screaming Mimi, a member of the Grapplers. Mimi was invited by Baron Zemo to be part of his Masters of Evil after the Grapplers were done, but was captured on her first assignment (round of applause). Many an attempt was made at being a villain until Mimi joined the Thunderbolts and changed her name to Songbird. Many members of the team enjoyed their new hero status, including Songbird, and turned on Baron Zemo. Once Hawkeye became the new Thunderbolts leader, Songbird became accustomed to being a true hero. She has been a frequent and heroic Thunderbolts member ever since.
Sandman
I'm still not over the fact that this guy, the Sandman, long-time criminal and a massive pain in Spider-Man's arachnid rump, was once an Avenger. An official one. He was a real good one too, turning the tide of an Avengers vs Sinister Six battle and taking care of a swarm of Doombots. Of course, he did return to his life of crime when Wizard warped his mind to bring back his nasty persona, but while he was doing good, he was doing it a lot. Sandman has been mentioned by developers occasionally, but they have said it would take quite the programming to make it happen. We'll likely get him as a villain rather than a hero, but the possibility is still there.
Anyone affected by Axis
The only point Carnage has been any kind of hero is during Axis, and he's a team-up within the game. Hobgoblin is in a similar kettle of fish, and he'll like become an enhanced costume for Green Goblin. So what's to say the likes of the Absorbing Man and Jack O'Lantern can't join the club? Probably because they aren't as popular as the former, but that doesn't rule out Sabretooth and Mystique, who have quite the following within the community.
Some good ones I missed because dumb:
Thanos
MODOK
Mysterio
Pyro
Silver Samurai
Silver Sable
Is there anyone I have missed? Who do you guys think should have some villainous heroism?