"The pickups are attached to a plate that can be easily swapped."
Why? Here's why: "The pickups are attached to a plate that can be easily swapped."
That's 1 example, probably the single thing that would attract more players than anything else, and there are plenty of good ideas in your guitar plans to go along with that. I like the idea of swapping out necks, too.
I'd want fingerboard options like: zero fret, 24 frets, scalloped, fretless, a fat vintage C profile and some other profiles.... If I had to lathe and mill my own necks, though, I wouldn't. I'd feel great about buying truly interchangeable parts, one or two at a time, from builders/sellers, as I could afford them and whenever I am in the mood to change something up.
I'd want to see more space on the frame for larger bridges, esp. Kahler, Floyd Rose, and other vibrato/"tremelo"/whammy bar bridges, and I'd need to be able to place a pickup closer to the briddge than 9 mm (to fit my Roland GK pickups). Those would be required before the modular axe could be useful to me.
I'd also want modular parts that let me make a solid, reliable headless guitar. Those things are really wonderful.