I think the thing that makes sense here, assuming Mastodon actually becomes a popular thing, is for companies and other institutions to run their own servers and limit accounts to their employees or members. E.g. nytimes or wapo or national review or whatever could run their own servers for their journalists. NBA and NFL for their players. SAG-AFTRA for actors and TV personalities, etc.

I know that there's a kind of verification mechanism already in Mastodon where you put a backlink in your website and then you get a badge. But it is an order of magnitude more complicated than "this server vouches for the identity of this account," and website ownership isn't really identity anyway.