Modern in the sense of looking like a smartphone/tablet, which ironicaly makes it easier to make work on a C64 than many of the user interfaces that were actually used on the C64, like GEOS.
It's quite fascinating that systems like GEOS with "proper" windows etc. looks more dated than a menu system similar to ones you might have found on a real C64 app (e.g. Koala Painter had a full screen full of rectangular "icons" to pick tools etc.)
One of the ones that impressed me at the time, was the GUI of The Final Cartridge III (the irony of there being a 3rd "Final" cartridge...) [1] which had "proper" windowing system in ROM with a look that was inspired by AmigaOS 1.x. Unfortunately pretty much no software supported it, so the GUI bit went mostly unused (wasn't typically why you'd buy it anyway - these cartridges were "freezers" used to pirate software and/or for their built in fast loaders and machine code monitors)
It's quite fascinating that systems like GEOS with "proper" windows etc. looks more dated than a menu system similar to ones you might have found on a real C64 app (e.g. Koala Painter had a full screen full of rectangular "icons" to pick tools etc.)
One of the ones that impressed me at the time, was the GUI of The Final Cartridge III (the irony of there being a 3rd "Final" cartridge...) [1] which had "proper" windowing system in ROM with a look that was inspired by AmigaOS 1.x. Unfortunately pretty much no software supported it, so the GUI bit went mostly unused (wasn't typically why you'd buy it anyway - these cartridges were "freezers" used to pirate software and/or for their built in fast loaders and machine code monitors)
[1] https://en.wikipedia.org/wiki/The_Final_Cartridge_III