Well, for one, BTAS wasn't all "dark and gritty", so, you're not really making a point with that one. And, also, how did adding a fake Batman meant to not keep it dark? Batman is inspiring people...that would be obvious.
I wouldn't care about a nerd heatwave, and also to the fact that Nolan has said he doesn't look at message forums and what not, so why would he care about that? Lol.
Having a female villain would just make the third film a lot less dark, as we haven't seen a good and decent Batman film that can handle a female villain, and plus a female villain in a dark comic-book series just as this, I can't see it working out extremely well. One reason why Nolan didn't care to add in Harley Quinn.
I'm all for no Catwoman obviously, and I have mixed feelings for Riddler. One day I think he would do fine, then the other I think Riddler would just feel too much like Joker.
I say 100% 'yes' for Black Mask though. I think he would be an amazing villain for the third film. The second villain, it's a toss-up. We could have another villain that uses his mind, such as Riddler, or perhaps a villain that could be hired muscle, such as Deadshot, or even a Bane(but obviously more realistic-based).