REPLs are basically shells. They behave the same way in every essential way. So the real question is support vs non-supported shells. But then that is easy - non supported shells fall back to just what a normal commands do ATM to process input/output. Other applications like TUIs are also easy to deal with as they already enter a different mode called raw mode - when a application requests that it can do what they currently do - switch to a new buffer and give full control to that one application.

I can’t think of any, but I’m not the most creative person… what do you have in mind?

Having smarter scroll back that knows the difference between a prompt/command and the output would let you do quite a few things that would be nice to have. Such as collapsing the output so you can only see the commands, keeping the command at the top of the screen even as other output scrolls off the top so you can always see what was running. Extra support for other UI elements could be nice to have as well - like tooltip support for blocks or similar.

All the shell - or really any application - needs to do is tell the terminal which bits of the output are witch. Like mark the start/end of the prompt, command and command output. Then the fallback is basically ignore the markers and print things out like it currently does.

And those are just random thoughts I have had over the last few days. These can be implemented in backwards compatible ways I believe and don't need special support for specific shells - just needs to expand the VT100 protocols to be able to send more information between the terminal and shells/applications that are running. Much like how color, cursor positioning, entering/exiting raw mode etc are already done. Though I think some tight specialized integration might be a good start to explore these ideas before the protocols are formed.