Jax would make a better rival to the protagonist. He's definitely to moral to be the antagonist, or even on the evil side of the spectrum for that matter, but he's most certainly no main character.
Evil would just be a take your pick situation. you could have a really good plot arch or saga with Fiddlesticks as the main evil. Noc, Veigar, Singed, Shaco, etc.
Akali, Kennen. and Shen would make great good guys, but not the main character. They are more like enforcers for good and don't seem to have enough depth to flesh out a protagonist.
Twisted Fate would make a great "han solo" character.
Lee sin would make an awesome protagonist. Plenty of depth and back story, room for growth, troubled past, etc.
Vongeo is right though. Ezeral does, unfortunately, make for the most fitting "main character."
As much as I hate Teemo, he also wouldn't be bad.