I was torn between the 5" and 6" options - I generally prefer a smaller device, and as others have commented, the aspect ratio needs to be taken into account in addition to the screen diagonal. I find the C2 too large and the X10III too tall (rendering it a bit weird in landscape mode). And please, please, don’t pick a display with notches in it!
As for internal memory, I think I’ve gotten along ok with what I have in the XA2, but a bit more probably wouldn’t hurt. I would prioritize an SD card slot over bigger eMMC - provided it is “fast” (not connected via some left-over USB2 interface or such).
Front RGB LED would be nice, but as others have already said I suppose using a corner of an OLED display would also work well.
I appreciate the convenience of a fingerprint reader and I really like the one on the back of the XA2.
Cameras: The primary reasons I don’t use the phone cameras are all more related to usability than to megapixels etc. Once slowness, focusing problems, and geotagging problems are solved, I wouldn’t mind a higher-quality camera. Possibly more than one main/rear camera, in which case I’d prefer one to be a wide-angle one. As for the selfie cam, most anything would be ok for my use (the occasional video call).
I want my phone to lie flat and have a strong dislike towards bumps ever since the first Pilot handhelds. In case (better) cameras need greater thickness, then I would prefer to keep the phone flat and to use the extra volume for a larger battery. I have another device, which is substantially thicker than the Xperias, and it is surprisingly nice to hold (due to the thickness and its chamfered edges). And speaking about the back: Please use just about any material other than glass. Make it textured to improve grip and hide wear. I don’t want to put a protector on the back in addition to the one over the display.
Reviving TOH might make this an interesting device. A place for a larger battery would be an “obvious” Other Half. Providing TOH with a second USB-C interface (in addition to the I2C I assume the J1 had) would open for keyboards, Yubikeys, or audio adapters (in case there’s no 3.5 mm jack) even while charging the phone.
Accessories: I would like to be able to order the phone with a screen protector, bumper case or other case. Borrowing the the outer dimensions from one of the more popular phones on the market would allow using protectors and cases more generally available.
Edit to add: Would I buy? It depends, as I don’t need a new phone atm, but I probably would if the price isn’t too discouraging and longevity of the device looks ok (battery, repairability). It would be an act of support for Jolla as much as a way to make sure I have a viable option when my current phone no longer works.