I don't think there's much to worry about. Having large general instances is perfectly healthy and good for the Fediverse as that's where people new to the Fediverse will land.
I predict that large niche instances will start popping up, one example already being programming.dev, and that's simply because there are domains where you might need extra customization.
For example, one can imagine a mathematics & physics oriented instance where LaTeX is available, or a chess-only instance where you'd have things like chessboard.js to allow members to post chess diagrams etc... Basically a return to what we had with old-school forums except this time the instances would be federated.
Just from my own subscriptions there's also startrek.website and dormi.zone (which is for the game Warframe). I think having an instance function like that is pretty awesome, if I want Star Trek or Warframe related content I know exactly where to go now.
For example, one can imagine a mathematics & physics oriented instance where LaTeX is available, or a chess-only instance where you’d have things like chessboard.js to allow members to post chess diagrams etc…
An interesting idea. But the problem with that is that the custom rendered content would not federate properly, so such communities would only really be usable to those on that instance, which destroys the whole point of being federated in the first place. Unless they were able to implement some sort of 'graceful degradation' so the content was enhanced on the main instance, but still serviceable on other instances.
Good point. I think it could still work well if the processing is done on the server, ie the specialized Lemmy instance processes the LaTeX equation and replaces it with a generated PNG.
Taking lessons from mastodon, I think server costs can really affect an instance's decision of how many users and how fast to register them. Can't blame them. lemmy.world just happens to have a pro admin who also runs mastodon.world.