[gaetgu]

As mentioned in another comment, most (if not all) of this can be handled by a shell. For example, I use fish, and with it I get highlights and visual autocomplete (with gray text, and explanations of what commands do).

[fathyb]

I meant something capable of extending the GUI. For example, displaying the autocomplete with documentation and mouse scrolling. Shells can do that, but they are limited to text.

I also want a marketplace where I can browse extensions made by the community, see votes and comments. I think that it fits nicely in a terminal that's like an IDE.