It seems like the singular great feature of a camera this size would be weight. Everything else is a distant second. I could probably use a 20x24 Ebony once...50lb? That's almost sack of concrete mix. My hat's off. I've toyed with the idea of torsion box construction to keep the weight down on a camera this size. The trick would be to keep the bulk down though. Hollow core doors are torsion boxes, if you ever lifted one it's amazing how light they are, essentially cardboard honeycombs laminated with very very thin plywood, but they're also notoriously easy to put hole in. In the end I'm glad I favor somewhat intimate contact prints, so I don't pursue it.
I think Ritter has performed a miracle of design with the carbon fiber 20x24, so it's certainly worth pursuing non traditional materials. Phillips' design looks like it could be parleyed into a 20x24 very easily. I'm not aware if he's ever made any camera bigger than 11x14.
Modularity seems straight forward enough- if the front and back are big enough you can adapt anything you want to them? Certainly no problem with any 20x24. But maybe you mean something else there.
Whenever I set out to make a camera for myself, the fun part is deciding what I want on it, and working out the problems in the design. I envy you this undertaking, seems there will be no shortage of interesting challenges. Good luck with the project.