Here are some things I'd like in a case that I'm yet to find, if you make one that does them all I'd buy one
I've not seen a case which fits the Pi with the Wolfson audio card (which is tall and has external ports) so you could have a lid option that's compatible with it? Also there aren't many cases for the model A (perhaps with good reason, it is a very niche product...) so perhaps you could have two options of base as well?
It would also be great to mount a PiHUB
on top of the case (preferably with it's bottom casing removed to keep the height down) and Ideally it would have a perfect length integrated cable to power the Pi, with the Pi end hidden to protect the socket while it's being transported and avoid accidental unplugging. This can't come at the expense of access to the GPIO pins though...
Talking of GPIO you could have an external header or mount some switches and LEDs on the side? Everyone loves switches and LEDs
I don't know about making it look good and being practical, as soon as you go away from a small rectangle you limit which ports can be accessed so I'll leave the aesthetics to you!