It would be a nice idea to have an integrated referral system/ extension that people can buy. I would like it if it could work by adding a new link under the account dropdown menu and also have multiple parameters for people to change. An example would be like this referral system from Mercari where, when new users sign up, they will be automatically assigned a referral id which allows them to send a referral link for their friends to join.
Also through the backend maybe they would be able to change how much users will get from a marketplace order when someone joins through their link and buys an item.