Can it work in background and interact with other characters being outputted?

[vi]

It would be even nicer if one could Ctrl+Z to continue issuing more Bash/whatever commands (or even start ncurses things like Vim), yet background Ternimal still lived and also perceived non-whitespace characters as walls.

[p-e-w]

Cool idea, but unfortunately impossible to implement. Terminal emulators do not provide a way for programs to read characters at terminal cells, so Ternimal cannot "see" those walls.

[vi]

So it can allocate a pseudo-terminal itself, like /usr/bin/script.

[p-e-w]

Yeah, but that would be a bit out of scope for this project ;)