Not to mention the reason netplay is disabled is it caused massive desync issues because of randomization in the core. the zsnes team is working to fix this, but from what i hear it's nowhere near ready yet.
Anyways as for the things i'd like to see implemented, most of them i've already mentionned and i'm pretty sure you are aware of them, so i wont reiterate myself to sound like a broken record.
One thing i've just thought about, and i don't think it's ever been mentionned, zsnes supports a slowdown key as a counterpart to fast-forward.
I know rewind is already there, but it can be useful is you just want to get thru some part whitout having to play it over and over, or really fun if you want to pull some difficult trick.
This feature seems to have been added on 5/19/05. so afaik that's about 3 months after the current wip zsnexbox uses. if its something that interests you, i guess would be something to look into when implemented multicore.
Main reason for this post is i'd just like to add it's been a great run, i'm gonna miss checking the xs emulators forum every day to see what new feature might be included in the newest release.
You are definately the coolest and most supportive / close to your users xbox developper amongst the many great people who have brought all this amazing software over to the xbox.