What features are a must have in a computer case?
Well really that varies depending on the person but IMO it would be:
Even a water cooled rig needs good air flow and that can be especially so when the radiators are mounted in the case.
As far as the drive bays go... I would say depending on where you want to put the reservoir at a minimum 2-3, 5.25" bays and the 3.5" bays kinda depend on how many HDDs' you plan on having not to mention SSDs if any.
also you would want to think about the orientation of the 3.5" bays as well.
just some thoughts hope that it helps!