I wonder if, in that same spirit, it's possible to make building, together , a complex microcontroller , a fun project ?

Well there is the FAP80 now. https://github.com/dekuNukem/fap80 https://dekunukem.wordpress.com/

That classes basically builds a microcontroller. It's Harvard Architecture with a split iROM and dRAM. Simple ALU, 16 bit CPU. You'd need one hell of a bread board to physically build it, though.