According to my (brief) research, the Magma Chamber was ditched in favor of the Crystal Scar/the Dominion mode. Still, though, it'd be nice to have another map for the classic mode. I feel like they add a lot more flavor and potential than, say, a new champion, since each map has different metas, a different pool of champions that are especially good, etc., etc. Basically, I think you get a lot more bang for your buck from a map than a champion, though both are certainly ideal/necessary (however you look at it).
Meh, just my thoughts, but again, it'd be nice to see a new map. I'd fancy a 4v4 map myself, but that's mainly because I frequently find myself playing with three friends than four, haha.