Why?
I like UNIX and video cards. I used real hardware video terminals (vt320, vt340) when I was in high school.  I was a software developer that knows a lot about software. But hardware was mysterious for me.  It had changed when I started tinkering with microcontrollers. Now I understand how these things works so I can make my own hardware for computers!

What limits?

I want to use one atmega328p (in Arduino Nano) with a few additional parts.

Is that new?
There is a lot of projects that outputs video signal or VGA monitors. Protocol is well documented.

http://www.theresistornetwork.com/2013/03/avr-vga-generation.html

Ben Eater: https://www.youtube.com/watch?v=l7rce6IQDWs, https://www.youtube.com/watch?v=uqY3FMuMuRo

Existing projects are far from useful UNIX-terminals. Popular resolution for terminal is 80x25 characters, but 40x25 was used sometimes too.

TinyVGA (http://tinyvga.com/avr-vga) can output Composite video signal with 38x20 charaters and VGA signal with 20x20.
VGAVR (http://jmp.no/blog/vgavr/) - 18x19

There is a ChibiTerm. Looks better than my BSW-100. It using STM32 chip. https://hw-by-design.blogspot.com/2018/07/low-cost-vga-terminal-module-project.html

This terminal outputs VGA signal with the best character resolution among AVR MCUs. 

What is different?

BSW-100 terminal can output 40x29 characters but with a single color and specially crafted font. It can be used as a real video terminal. Characters typed on the keyboard are transfered to PC (or router).