The Addon API
Users will be able to upload their own addons (models) and will be able to use it ingame
Currently, users can upload models without scripts/localscripts/modulescripts to any multiplayer instance of my game. Inside the singleplayer experience though they can use whatever model they want because its singleplayer and who cares.
Of course this may change in the future but right now I want to see how the community does with this.