So, basically,
X-Men?
Okay, so, looking over the list, here's what I have to say about each one to help you decide. My favorite concepts are in bold; the rest is just commentary.
Flare: Unfortunately, whenever someone thinks of elemental manipulation, one of the first things that come to mind is the ability to manipulate fire. It's really been done in basically every mutant superhero story that's out there. I mean, it's
possible to pull it off (which is why so many people do it), but it'd be more interesting if you went for angles that people don't normally explore. It'd give your story a fresh edge to it.
Sage: Same as above. A lot of superhero stories have magic users. On the other hand, I'm looking at the phrase "teleport discs," and I'm thinking that it might be technological based or otherwise not actually an innate ability. As in, if this kid gets outside help to use his powers (and just happens to be really,
really smart), I'd read a story with that kind of character in an instant. However, if you didn't mean to imply that, it might still work, just because sorcerer heroes tend to be kinda awesome. (Doctor Strange, anyone?)
Augment: A supporter character who
isn't a girl? Yes, please. (I think of Ando from
Heroes here, who I always thought of as awesome. That and I like the idea of a superhero whose powers are basically useless on their own. It encourages teamwork and/or hilarious situations involving attempting to patrol alone.)
Venom: I like this idea in part because poison emission isn't a power writers normally explore and in part because you usually see this attached to villainous characters. (Hi, Poison Ivy.) It'd be interesting to see what kind of backstory you cook up here because you certainly have every opportunity for it. (Just so long as, you know, you're not basically copy-pasting Rogue's history here.)
Speed: Again, it's a power that you usually see in superhero stories. The Flash, Quicksilver, Superman... or, hell,
everyone here. In other words, you encounter the same problems as the ones you'd hit if you were using fire manipulation.
Iron: Normally, I'd also be telling you about problems involving superhuman strength, but the difference here is that this is a chick. You don't see that that often, and when it's done, it's usually either for the lulz (She-Hulk, She-Thing) or coupled with being completely effing psychotic (Niki Sanders). If you manage to play this straight, it'd make for a really interesting character.
Feral: So long as we're talking about mimicry and not transformation, you've got something that not many writers have been able to do without doing it for the lulz. Just avoid going overboard and interpreting "animal mimicry" as "every flipping power on the face of the earth." While I know you're writing about superheroes, you'll want to create limitations for yourself that help you to shy away from pumping the fic full of Gary Stus.
Giant-Man: Again, not something people have been able to do without being for the lulz, but it can potentially be an awesome power
(or, alternatively, an opportunity to make Ultra Man jokes at every turn).
Blackout: The only reason why I shy away from this one isn't because it's overdone (although it's not unusual for people who try to make a team of elemental heroes to have someone like this... and I can think of one popular example whose name starts with "R" and ends with "aven") but instead because I feel like these kinds of powers tend to be attached to characters who are stereotypically dark (either former thugs, gothic, or just really, really angsty). If you were attaching this to someone who thinks "the world is made of LOVE AND
PEACE RAINBOWS," it'd be a different story.
So, yeah, there you have it. My two cents about the concepts. The whole idea itself is pretty interesting, so I hope this helps, just because I'm interesting in seeing what you do with it. Good luck.