For a Haiku based implementation, what about:
Each command returns a BView that can be added to the window. For simple commands it will be a BStringView with the result. But it could be a more complex thing with images, buttonr, whatever.
It could be made to look like a chat app at start, with your requests and the replies clearly separated in chat bubbles or similar. I imagine there would be an “help” command, that would reply with a tree view of possible commands, which you can click for more details.
It could have legacy support for running executables from the path, but maybe some things would rather be done as add-ons