A smart sound browser
A smarter instrument browser would be nice, with descriptions of sounds in the factory sound bank for searching, I.e. soft, mellow, hard... Also midi grooves built into reason would be awesome. That way you could easily add drums or piano chords like the toon track ezkeys or ezdrummer. Then we could adjust as much as we want after the preset addition.
Also iPhone or iPad support would be awesome to use in conjunction with parameters like you can with omnisphere.
A sequenced long would also be cool. I love making drums with Reddrum, but kong is so much superior.