The Watcher gives you the opportunity to convert some Marvel villains to heroes, but you have to sacrifice an equal number of good guys to the dark side to maintain the cosmic balance.
You can swap as many or as few as you would like as long as there are an equal number of heroes and villains replacing each other.
Oh, and the definition of villain and hero are open to interpretation, so don't get too worked up if someone thinks your hero is already a villain or vice versa. With how comics are written they've probably all done some bad or good at some point.
I will start things off:
Villains to Heroes:
1. Mysterio
2. M.O.D.O.K.
3. Juggernaut
4. Monica Rappaccini
5. Baby Thanos
Heroes to Villains:
1. Rocket Raccoon
2. Honeybadger
3. Iron Fist
4. Cloak
5. Gambit