There's no fighting game for the A Certain Magical Index/Scientific Railgun series, and there really needs to be. I mean, the closest thing we have to this is a fighting game called Dengeki Bunko Fighting Climax, which is currently only available in Japan in Akihabara, with a possible console and/or PC port on the way, which hopefully will make its way over to the States.
(This game has the 2 main protagonists, Touma and Misaka, as playable characters.)
Still though, we need a fighting for these 2 series (they're set in the same universe). Both parts of the universe bring forth excellent characters that would make for a great fighting game. Make it so game developers, and you will have all of my money...er well...all $50 of it.
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The AoT game for 3DS is most likely going to suck.