I'm not the GM and ultimately it's his decision and will be based more off the concept you come up with anyways, but as a player it makes me a little wary personally. In my experiences If not handled carefully that kind of thing can lead to mary sue-ness and plot hijack, sometimes totally by accident too. I mean right off the bat (huh, first Bat-pun of the RP) you're changing the backstory of a major character so that they have a previously non-existent kid which might be out of character, and you're coloring a lot of interactions that would happen with that villain and making it difficult for any plot involving them not to mandate a whole lot of focus on you.

Granted, this RP takes place in an AU anyways, and considering most of us get our own episode(s) in the limelight plot hijack is a LOT less of a concern so really I'd say a lot of it is what you have planned, how well you can write up your concept and how well you handle the character. That and what the GM has planned for said character.

Just my thoughts.

Also in case it was part of said character concept, we aren't really allowed to do villains. Villains who are redeemable and might be permanently/semi-permanently good-ish guys like maybe the TANDEMs might be okay with GM approval though.