A horizontal MB tray would be nice instead of the usual vertical MB case like almost every case out there.
Space for a triple up top, a dual up front if you remove the HD cage or on the bottom, sort of like what the CM690II does. This way people have options.
A fillport on top would be nice, make it delrin and not aluminum.
Places to route wires like the Corsair 800D.
Behind the MB tray should have 3/4" for cables to go without making the side panel bulge.
Figure out a way for the 5 1/4" bays to secure drives and stuff without the metal supports so a res can be put in without cutting them out.
If you really want to get fancy, put pass through PSU connectors on the MB tray so that the PSU can connect to them and then extensions would go to the various places.
SATA backplane.
Integrated fan plugs/multipliers where the rads will be.
Integrated temp monitor display on the front top that excepts standard temp probes like BP and Koolance.
Fan controller that can handle at least six fans for the rads.
Make the doors swing open instead of just popping off.
If you do a window, make it so that it only shows the MB area.
Graphics card support like the 690II.
Quick release PCI slots hold downs like Lian Li does in their cases.
Don't make it outrageously priced like the 800D and other cases.
Removable filters.
If you do a triple at the top, it would be nice if the rad could be mounted to a removable mount that could then be lowered into the top for easy mounting of the rad.
Looks wise, more curves and less corners.

That's all I have right now.