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?
There’s a few niche art websites where ads are from the artists within the website itself! Its less invasive, its people giving to a community they love, its always more or less related to the website itself. Not saying I love ads either, but this way, its fine I guess.