New Systems
Commander X16 would be an easy system to add - lots of documentation and development https://www.commanderx16.com/forum/index.php?/home/
Microbee (Australian Z80 Computer) - has an emulator https://www.microbeetechnology.com.au/ http://nanowasp.org/
BBC Micro (Large user base in the UK) - has an emulator http://b-em.bbcmicro.com/
When it comes to Commodore-inspired computers, apart from Commander X16, there's also MEGA65 (https://mega65.org/) in the works, the time will tell which would become more popular among users. :) And on the Sinclair front, ZX Spectrum Next (https://www.specnext.com/) seems to be doing pretty well for a homebrew platform.
BBC Micro is a nice classic platform that is definitely worth a closer look.