First: there's no way any one poll on this is going to be all-inclusive of what it takes, so feel free to comment and maybe subsequent polls can revise and narrow the list. When the thread title says "solo(ish)" it doesn't preclude a character joining a team, even early on (e.g. Black Panther or Booster Gold) but focuses on those heroes created as solo entities, even if only in the internal logic of the story (Iceman or Negative Man probably wouldn't qualify). It also assumes the character is meant to carry a title, or at least a feature in a title, if only eventually so.
Aside from a good writer and good plots, what does it take?