The only way to make any character viable is to commit to regular publishing--even if it involves restarts--with talented creative teams who are interested in developing characters and expanding mythologies. Keep doing that, and eventually you have a character with a library and a mythology. If you don't sell what you intended, re-evaluate and re-try with the same character until you get it right. Put some thought into the character and what role they can fill that isn't being filled. Treat them like you do your other big guns. If the character isn't bringing anything new because there's another tech guy or detective or speedster on the varsity team, then that's the wrong character to push. If the character isn't powerful enough to stand with the other members of the varsity team, then that's the wrong character to push. If the character is going to be talked down to in order to maintain the relevance of another character, then that's the wrong character to push. Imagine this character has a fanbase like Superman, Batman, or Wonder Woman, and think about depictions that would tick off those fanbases as well--then avoid that. Also, test for universal appeal-- Would this character and their issues be relate-able or interesting to someone in China or Norway or Ghana?