if you do not care if other people can run it then go
Maybe I put it a little too harshly. The intention is to write true Amiga software that goes further then what's currently available, especially on lower end 20s and 30s with some fastmem. If that's successful, it shouldn't be a big deal to add some chunky GFX support for GFX cards.
The emphasis is on OCS/ECS/AGA+20s/30s, because I believe these machines are capable of more than what we're seeing today. All that's available is old desktops and old GUIs, and this can be massively modernized without any crazy requirements (obviously some of the eye candy will be missing, but that's not what makes a modern GUI system modern).
All of the low level graphics code for this will be planar blitting routines, some c2p, and a hardware sprite for the mouse pointer. To get this stuff to work on GFX cards would require adding some code to access the GFX card functions, add chunky versions of the planar blitting routines (which are vastly easier to write than the planar ones) and to not use the c2p. Certainly not a massive task when you have an actual system running. It's just something I wouldn't do from the start if I would take on a project like this.
As for the 68k assembly language, that's not negotiable
It's a hobby after all