Also, I'm trying to look further than simple, one-time music mixes, as if all goes well, we could even have many other things on air, like interviews, "talkshows", news and such; just like a real radio station. But we'll see about that.
I highly encourage this. I would love an elements radio that also included forum news and interviews, especially during War and other major PvP events. The opportunity for propaganda is fantastic.
On topic. I for one am generally a fan of any sort of music that is thoughtful and creative. As generic an answer as that really is, it sounds much more eloquent than the oft repeated phrase, "The only music I don't like is country and rap".
For elements, I think instrumental music would be best, if we're just looking for music to play the game to. I find lyrics and spoken word to be distracting when I'm trying to focus on a task; pure instrumental compositions, however, make it easier to get into "the zone", while also breaking the tedium of mindless grinding. Thus, I'd suggest some sort of orchestral work, both classical and modern, and some form of electronic music. I'm not quite familiar with the idiosyncrasies of different electronic genres, but I assume that you know what I mean when I say that. I'd only request a ban on skrillex.