My personal and never-asked-for wish is fairly small - Backlit buttons. Id like the ABXY and Steam/start/select/Menu button to have a faint glow to them with a toggle on/off/dim in the menu. Maybe even the analog stick could glow where the current sticks are white.
For the gen pop I think the low hanging fruit of upgrades people would actually be interested in would be:
Battery upgrade, or some kind of switch-like efficiency upgrade that makes the battery much more effective
Some kind of screen improvement, be it smaller bezels or an OLED option or even both.
A general change in the ergonomics based on user feedback. I personally like the deck as is but Ive been surprised by stuff I didnt notice before
Beyond that its going to be niche improvements that some people may find interesting and others not so much:
SD and USB slot changes (additions, change in position)
Various internal changes with the hard drive or upgradability options