You mean, like Captain Cold? lol
Captain Cold has what, 3 fans on here?
If you want to go by pure fan favoritism, the roster has that already in spades. The roster is stacked, we can afford a lesser-known. BUT, for the sake of having all the fan favorites in there, and we are judging by fanbase, you do realize that's just going to lead to *gasp*, more BAtman characters right? I've seen more desire for the likes of Scarecrow or Ivy than Captain Cold. Or heck, even beyond Batman characters, you have Swamp Thing as a top contender.
Fact is, not every fanbase is going to get what they want. The smaller the fanbase a character has, the less chance they have of making it into the game. You can't include everybody.
Also, never meant Grundy was obscure. I was implying that people are quick to judge someone, just because they aren't Mr./Mrs. popular. Grundy was one of the lesser liked/appreciated characters in the game, and after more of his was shown, people fell in love with what he had to offer. She could do the same.
As for Killer Frost...who are you to judge how big her fanbase is, or the lack there of?
http://testyourmight.com/threads/killer-frost-let-me-draw-the-heat-from-your-bones-the-fire-from-your-heart.28186/
That was a thread dedicated to her inclusion. And i'm sure that's not the only gauge of her fanbase. So I wouldn't be so quick to say she has no "REAL" fanbase.
At the end of the day, it's all about preference. The character someone wanted wasn't chosen over another character who did. That's innevitable. I wanted Dr. Fate. Doesn't mean I am unwilling to give her a chance, even though I harly use female characters. I'm still interested to see what her gameplay will consist of.
They pick the characters for the reason. She obviously fits a criteria that another character doesn't.