LambdaMOO was the former: you were using existing objects to code new objects, by sending the existing objects messages on a live server. (A bit like writing bash scripts on a multitenant Unix server where everyone can drop things into /usr/local/bin, really.)
I feel like there’s a wide-open space in the market right now for a game like Minecraft or Mario Maker where everything is built out of “blocks” with behaviours—but where these block-type behaviours are scripted within the envrionment itself, so you can create and share new “block types” without the need for any meta-VR commands. Like a Steam Workshop service, but one that is intended for players, rather than for game developers. A game with an scientific invention economy.