The big thing missing on repl.it is to use visual studio code instead of their own editor (think like Stackblitz), that would really be a game changer to be able to use one custom key-shortcuts, to use vscode plugins and so on.
We decided against using vscode verbatim for a few reasons, chief among them is keeping repl.it simple for people starting to code.
And as much as I like vscode and the team behind it, it doesn't seem smart to tie your core tech to a project under the control of MSFT.
With regards to extensions you can easily imagine a vscode-replit bridge.
Finally, we came up with a novel approach to IDE/window management that we think people will be excited to write extensions for when we open it up: https://repl.it/blog/ide
I'd love to see an approach similar to Makepad for the Repl.it IDE (all powered by WebAssembly). Keep up the good work!