Yeah, but you can't have it both ways. If you want them "available", they will get dusty (which you said you didn't want by hanging them up), unless you put up a glass door, which sounds fantastic, although REALLY expensive. My thought was you own little "bass room". With all those conflicting desires in mind, I suggest three alternatives:
1. Do you have a full wall - several years ago, I made a full wall built in bookcase. I would suggest the same. Either have glass doors, or a curtain, with fabric like this
You could build a shelf over them, which would both cut down on dust and give you storage for the cases. No matter what, any build you do to have all the axes will be expensive and take a big part of the room.
2. Compromise - do you really need ALL those basses out at once? Build a smaller storage case - say with 5 or 6, Rotate them in and out (and quick dust when you change). Keep the rest in the closet.
3. The cheap way out. Get a bunch of those multi bass folding stands. Line them up against the wall. If you don't want the basses to get dusty, put a cloth over them (fabric above or similar). Less money, nothing permanent.
I still want a bass - even a cheap import (c'mon - you'll never miss ONE).