Plan9 “terminals” were like that. Create a window, and by default the text shell runs in it. If you have vdir installed, and you run that in the same window, you get a semi-graphical file browser. Exit that and then run games/doom and now doom is running in the same window. Exit that and “cpu” into another machine and run riostart and now that same window that did all the other things now is running a window manager on the remote machine, displayed on your machine. Graphical apps, textual apps, everything. All in Rio windows. Smoothly, too. (It is a very different paradigm so I am not going to profess that it is user friendly or anything, but it does work, and it works well once you get your head around it.)