FLOSS: https://en.wikipedia.org/wiki/Free_and_open-source_software
I was thinking for exemple, why not modifying one of the Kenta Cho games and make a 64x64px version of it, it would probably be fun!
(As far as I know Kenta Cho games are rarely modified, usually only packaged as is, I for example packaged https://fr.wikipedia.org/wiki/Noiz2sa for Mageia)
I saw a modified version of Noiz2sa to use darker colors, or to port it to SDL2, I'm not aware of other derivative work of it.