Some ideas...
Multichannel
Full envelope Control
Advanced Realtime Effects
(Phase shift/Echo/etc etc)
Which can all be be combined
(For example Pitchbend with sample offset changing with phase shift and echo :-) )
Support for Synthetic as well as Sampled instruments
(With control over waveform/ADSR/and more - like in ahx)
Multitasking, but still using asm for some parts making it fast and useable on 680x0
XPK/XFD/XAD Support

AHI Support but with also direct drivers for toccata output etc

That will do for now :-)
Perhaps you can keep me posted on progress buzz@totem.fix.no . Also, maybe you should post this to exotica forum as some music lovers there who might have some good ideas for you
http://exotica.fix.no(Although server is being upgraded right now, web might be down for some hours)