I'll agree with Ken that the decision to embrace 5ths tuning is one that means no looking back. You've basically got to lock yourself in a room for however long it takes. I've been trying to do that on a chamber bass I made in high 5ths tuning (GDAE) because it's small enough that I can fool my brain into believing that it's a totally different instrument. On the bigger basses, it is discouraging to realize how hard old habits die, if they ever die at all, but you'd need to talk with guys like Joel Quarrington or Paul Unger or other fifths-tuners to get a more detailed recounting of what the switch demanded.
As for the rest of it, my feeling is that if you want to make a bass for the 21st century, you need to step back and reconsider how a bass should sound. For me, I don't want a massive and cavernous rumble from my bass; I want a clean and focused sound that follows the listener and finds him wherever he's sitting in the back of the hall. I can probably numb your skulls with the many various acoustical reasons that underlie such a bass, but a good ****ogy might be from guys in my age bracket who remember pre-stereo "high fidelity" speaker systems.
Back in the day, to get a convincing bass in the lowest octave, you needed things like a monster folded horn. If you look at audiophile magazines of the era, you'll find projects where people used 30-inch woofers and turned their basements into part of their sound systems. No kidding. It gives literal meaning to the idea of shaking the floor.
As time went on and technology improved, you could get the same result by using a closet as the resonating chamber for your system. You will also find pictures of guys who mounted 18" woofers in closet doors.
Time marches on. Woofer enclosures came down in size to six cubic feet, to four, and then to three. With the advent of the acoustic suspension theory, we were able to put entire speaker systems on a bookshelf. You will not find people using closets for woofer enclosures any more. I have a subwoofer in a 1 cubic-foot enclosure. It has a single 8" speaker that pumps out bass at 32 Hz at a volume that'll make your bowels move (
needless to say, I dial it well back from that setting!).
Basses aren't loudpseakers, of course, but the idea is that we've gotten stuck in our thinking about basses. Bigger doesn't necessarily mean better or even lower. The demands of modern music are exceeding what can be done on what is basically a 16th-century gamba. My first question would be whether players would switch from their conventional basses to one that was smaller and had lower ribs if it would otherwise do as much as well as their normal bass. Next question would be whether the players would consider switching if there were trade-offs; that is, they had to give up some things they liked on the old bass to get even more (but different) things they liked on the new bass.