There are a handful of projects for making vim available in any textbox. They work by having some kind of hotkey that opens a temporary file in vim and then pasting the contents of that file into the original program when vim is closed. Examples are vim-anywhere, nvim-anywhere, vimlauncher-kde, probably others.

If you like vim, you could just use one of those methods to do everything you described. If not, you could probably use one of those as a starting point and just change the part that calls vim to call some other text-generating program or custom repl instead.