The biggest one would definitely be multiple audio layers. At least two, so you could have persistent music in the background with action-based sound effects over top of it (a sword clash, a door slamming, maybe even voice work, etc). The more layers the better basically, but I can't really see really needing more than a few for anything I have in mind, and just having two instead of one would make a major difference for me. It doesn't seem to me like too big of a jump from what there is now, but I don't have any idea what I'm talking about, heh.
Being able to fade in and out and crossfade would be really nice, too. You can obviously already make the audio file itself have a fade, but then you can't loop it. You can kind of get it to work by having two files, the same except one has a fade and one doesn't, then playing the one with the fade first, holding the game up until it's done, then playing the loopable one and looping it, but then the player has to wait for the whole first file to play. Though this could be helped by just having a "play one time, then" sort of a deal that you could put scripts into.
What would be super cool would be something sort of like the idea of scenery is with objects, where you could start multiple sounds at once and toggle whether or not you could hear them. Then you could build musical arrangements dynamically as things happen, like having battle music where a dramatic melody cuts through when you land a critical hit, or dropping to just percussion when you get low on health, and everything would stay synced up and in time with everything else. Something like that might be too demanding in terms of processing, though, even with compressed audio.
Sorry if that was too much all at once, I've had these things on my mind for a while.