I can understand that and it's a totally valid reason, but in my opinion it's not really a big deal. It's pretty much just aesthetic. If you took the exact same moveset I'm thinking for Killer Frost and gave it to someone like Terra (just replace "ice" with "earth/rock") nobody would complain about redundancy, but it would be the exact same thing for all intents and purposes. When I look at the situation in that way, it just seems like such a minor issue.
While we're mentioning it, I think this game could use an earth-based character. That would be badass. A fire-using character too, but I don't know many DC characters that use fire. Do they have anybody like Human Torch from Marvel comics?