I assume this is for DevTerm rather than GameShell?
Keyboard/Gamepad
I think a new keyboard/gamepad module would be great, for me it is actually a really comfortable device to hold to play games but the direction keys are almost unusable, having something much more traditionally shaped with a much shorter travel would be great.
As for the keyboard I do like it but with it being so flat it is hard to differentiate the buttons sometimes and sometimes make accidental presses of neighbouring keys, I think some shaping to the keys could really help.
On the same topic of the keyboard module, I know some people have done modifications to the trackball by replacing it with a different model (such as this one). Alternatively maybe consider a “nub” mouse/pointing stick like on Thinkpads.
Extension Modules
Extension modules would be really neat, I was actually a little surprised to find that the actual “printer” part of the thermal printer is (semi-)permanently attached to the DevTerm which I guess means that any other expansions that might come out would need the printer to be removed entirely to work? I expected there to just be some kind of edge connector in there.
It would be great if it was more generic and allowed you to plug all kinds of things in (shoutout to @fip and this thread about a cartridge module).
Some expansion modules off the top of my head would be things like adapters for 8 and 16 bit console controllers (think SNES, MegaDrive etc. - a bit like this) and a MIDI interface board,
New CPU
A low powered x86 core module is the next obvious addition I can think of (Dreaming: The DevTerm-X86) for an additional core module.
Also I’d love to see a more powerful RISC-V CPU at some point, I’ve recently backed the StarFive VisionFive2 with a processor which wants to compete more directly with the ARM SoCs in the Raspberry Pis.
Operating System
Officially supported non-Debian based Linux distributions would be nice.
Maybe something non-Linux like FreeBSD on the “easier” end of the scale and at the other end RISCOS would be amazing (but I suspect difficult and not very realistic but I want to give a shoutout to it anyway 
(when I can finally get a CM3 or CM4 I want to give it a try but I strongly suspect it will have all kinds of issues with screen size, mouse support and no wifi making it fairly useless - the brief did say “dream free and wild”.)).
New screen
Adding touch sensitivity would be interesting
“Other”
One other small revision I would like to see is to make it a little easier to swap the core module. In particular I like to play around with the R01 but always go back to my A04 for more “normal” stuff. I was thinking that a door on the bottom of the case to access the mainboard would be great.
Sorry, I may have got a bit carried away…