Freely modifiable and designable GUI framework/library/whatever for Arduino?

Maybe have a look at this AWind: GUI library.

However I am firmly of the view that GUIs should be built on more powerful computers and the Arduino just used for the things it is good at.

As well as all the reasons put forward by @mikb55 it is much easier to write GUI code on a PC with an operating system and access to easier-to-use programming languages.

...R