Follow the strategy of games like Fortnite and League of Legends. They offer extra cosmetics in exchange for money.

If you haven’t played these; things like “skins”, or armor packs, in game dances, etc. Are offered for purchase. They are explicitly for cosmetic enhancements and not game advantages (IE: changing your characters color won’t make you have a higher attack damage)

If you go large scale you could also do what battlefield did and rent out private servers, where people got to host private games with their customized settings.

Lastly, this one I don’t agree with as much, but you could offer quicker lobby times/higher priority in queue.

Ps: You can also do DLC’s with extra maps or something.