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?
I think a “selling” point, or rather, sponsorship point, is identity. You get a good social handle, and you are insuring that they will have a place to post their stuff. Like things like linktr.ee for example. People use that for their digital “identity”. Mastodon is like that.
Now it would be great to see tools to allow people to customize their own profile page more. I’d be in support of that.