I've seen a huge Omnisphere request topic here earlier. Since I couldn't find it, I made a brand new one. Sorry for this.
What I would like to see Omnisphere do is this: You play a note with leagto on, and instead of retriggering the sample when you play the next note, the sample will continue to run from where the previous note ended. Only the pitch changes. I think that could be a cool way of making evolving sounds.
Other requests that I'm sure have been mentioned a gazillion times befor: Let us import our own samples. And how about releasing some soundsource expanders? (Not that Omnisphere would need it, since the core library itself is so huge)