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?
This is what donation drives are for.
On the instance I am on the admin currently have collected enough last donation drive to run the server for three years.
Mastodon is an open source Free-As-in-Freedom non profit project. That is the POINT of it.