A good solution I can see is "choose an interest", with each public mastodon instance having some cheeky emoji and tagline in a list. So if someone typed "makeup" they might filter to the mastodon instance "beauty.social", instead of needing to find beauty.social before even getting started with creating an account.
The risk of this approach is that you are not picking an interest. You are picking an environment with a set of rules and regulation, an environment that has certain physical constraints (response time, scaleability), an environment that decides which instances you can federate with.
Try explaining that with an emoji and a fun tagline.
A person typically have multiple interests. Associating yourself with a single interest is usually a bad idea as sooner or later your would want to talk about something else.
It would be way more natural to have interests as channels for a person's account. People could subscribe to only particular interests instead consuming the full stream of the person's posts. I'm not sure if Mastodon supports this, but it's sorely lacking in Twitter.
