I wish there were some cheap FPGA version of the 3rd party implementation of the planned successor, the TI-99/8.

It was called the Geneve 9640 from Myarc:

https://dressupgeekout.com/geneve/

http://www.mainbyte.com/ti99/geneve/geneve.html

Wikipedia has a decent article:

https://en.wikipedia.org/wiki/Geneve_9640

12 MHz un-crippled 16-bit CPU, 80 column text, 256 colour graphics, up to 2 MB of RAM.

That would be much more promising for a Unix-like OS!

They are extremely rare these days, but a cheapo emulation would be great fun -- it's able to run most software for the 99/4A.