What I'd really love to see instead of all of this is an OS that has composable areas that can interact and have event loops of their own. E.g. ditch the app boundary and compose. We're getting there sorta with AI Canvas type things (e.g. github copilot app, some of the stuff in chatgpt visualizations), but all of this sort of work tends to devolve to browser type things instead of simpler components. This is mostly unrelated to the terminal side of things though from my perspective on this. But it's a fun thought experiment.
X11 supports this.
That was X11.