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?
If someone develops a good plugin or other kind of extension for mastodon that makes this possible, maybe the people that want to use it, can.
I don´t personally think it´s a good idea, nor do i think it´'s non-invasive, but why not experiment.
Asking mastodon´s main developer will probably not lead to anything, cause it would result in backlash from too many of it´s users. At least that would be my guess. But keeping it a third party thing, that integrates easily with the existing mastodon code base, will offer a good proof of concept.
Implementing anything involving payments is dangerous, doing it this way could offer a playground for server admins to mess around with. The ones who are willing to testdrive it could do so. In the meantime, bugs can be ironed out. Who knows, if it´'s a success, maybe then it would be a good idea to approach the developers and have something to pitch :)