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?
The elaberate answer is: Microtransactions for an open source Free As In Freedom project is absolutely frakking insane. I must assume the OP is a troll from Twitter delibertely out to destroy the foundation of the Fediverse.