The mod community for Rocket League has yielded some impressive results although it hasn't necessarily been bustling. Either way it's getting a boost next month as the game will gain Steam Workshop support, accompanied by a Steam Workshop Uploader Tool, allowing modders to upload custom levels.
The move means content creators will have an easier time uploading their work and getting it recognized, and users will have an easier time finding and installing it. Win win.
No exact date has been set yet for the update, but Psyonix says it'll have more information soon. Stay tuned.