Great topic!
Here's are some ideas (or wishlist ;))
1. Variable stick tension via easy spring swap mechanism.
2. Different stick designs modelled after real aircraft, swappable (like Cougar/Warthog.
3. 2/3 clip-on palmrests of different thickness to cater to different hand sizes. Similar to what X65 has now, but it comes with only one.
4. Dual throttles with easy removable idle and afterburner detents.
5. Different throttle grip designs, modelled after real aircraft, swappable.
6. Wireless, similar to logitech's Unifying usb thing
And now for the really crazy idea:
MODULAR DESIGN.
Basically the idea is that u buy the basic throttle and stick base, choose between either force-sensing module or spring module, then finally choose the throttle grip and stick design. The force sensing/spring module shd be easily assembled into the stick base.