Cosmetics.
Let’s offer paid cosmetics for users of your instance. Animated or custom banners, posts, new themes, etc.
It’s a model that works really well for Discord and League of Legends. Instances offering to sell aesthetic luxury to their user base could work for Mastodon too.
It’s non-invasive, it’s optional, it’s a great addition for some people, and it allows to raise funds to keep the hosted instances up and running.
Do you think it’s viable?
While I can understand a desire to try to make money off of running a Mastodon instance – consider that you can host your own private Mastodon instance for about $5/month, which gives you effectively unlimited control and customization.
If you intend to charge $5/month or more for cosmetics, what additional incentive would users have to use your server instead of their own? If you charge less than that, are you sure you’d be making enough money to cover the extra maintenance and responsibility you’re taking on for it?