@eli Same here; it's fun just dabbling around. My client work consists primarily of web development, so playing with hardware and low-level programming is a nice contrast.
For some reason, my projects often revolve around the Game Boy. Its CPU core is a mix between Z80 and the Intel 8080. I guess nostalgia plays a role, but I do love that system.
I've followed Rekka's and Devine's adventures for a long time, so Uxn has been on my radar. I've yet to play with it, though. But I've actually read the paper you linked to. I stumbled upon it when looking for small CPUs to implement on an FPGA (I have a TinyFPGA BX).
RetroForth is new to me, so thanks for the tip. A new rabbit hole for me to explore. 😊