In no particular order:

radio buttons still basically unsupported - no way of grouping them.

Add optional astropy units for values, maybe extending the tuple syntax: ('__a__', u.km). More difficult than it seems, because one needs to parse input strings. But very useful in a scientific environment.

'with' syntax still very limited, in particular locals() and globals() refer to the Gui class scope so for example anything imported at the beginning of the script is not available inside the with block.

Unit tests almost completely missing, also I have no idea how to properly test GUI code.

Meta-level: add this list to github :)