So, did you consider Lisp or maybe Smalltalk? Plan 9 or Inferno might also be options.
Plan 9 comes in different variants, the "classic" one (with a Raspberry Pi port by Richard Miller) or 9front, an Inferno porting tutorial can be found at https://github.com/yshurik/inferno-rpi
Lisp and Smalltalk can run with or without Linux underneath, e.g. on the Raspberry Pi.
Bare-metal Lisp is available with interim: http://interim-os.com
Finally, bare-metal Smalltalk is available in my crosstalk system: https://github.com/michaelengel/crosstalk
Of course, Lisp and Smalltalk can also run hosted under Linux, e.g. using Squeak (https://squeak.org), Pharo (https://pharo.org) or InterLisp (https://github.com/Interlisp/medley).
Or - a crazy idea - build an emacs-only machine. That would be fun! :)