I'm a singer and guitar player; I've been using Line 6 POD since 2004, I also have a variax 500, and I love it sound and signal flexibility but it's not so flexible in hadrwere desing, like all multi-FX in market:
I like to press the switch of my pod with my left foot and use the expresion pedal with my right foot but I can't do that because it is merge together, so I have to put my pod in the right or left side of my microphone stand.
If you desing a modular pod, with a separate expresion pedal I'll be able to:
Left the expresion pedal at home, if I don't need it.
The pod will be more handable and more easy to carry and find a case. The HD 500 it's very large.
Put my pod on desk and put the expresion pedal on the floor.
Set a better disposition of POD on stage. I usually perform my music in little stages