A few ideas
I would like to see improved playback options, for instance it would be good if we could have VST support but i understand the difficulties with this. It would be good however to be able to use virtual MIDI cables such as MIDI yoke to connect to stand alone samplers and plugin libraries. On the same note, it would be nice if there was a feature (similar to that of cubase's new VST expression) where by a user could define key/channel change commands for various dynamic and technical markings for use with KS or Channel switch sample libraries.
For example if a particular sample library used the note C0 to keyswitch to pizzicato violin the musescore user could define somewhere that whenever pizzicato is assigned to a violin instrument the C0 note will be sent on playback. This technique could be applied to pp, ff, mp, arco, etc.
I am also curious to know if musescore will ever be 64bit? this would give it a huge advantage over all other major scorewriters on the market which are currently 32bit and hence limit the quality of playback through sample libraries.
Hope that makes some sense :)