Personally, I see decentralized IDs being a big one, one accepted account that can let someone log into multiple servers as the same person. That'll lower the difficulty on choosing a server.

The other one, and this one I think may be controversial, but more and better feed algorithms. People want content that is relevant to them to be served to them automatically. Now we're FOSS, and not ran for profit, so we can do even better a give people control over their algos, but I think most people would rather just click a couple interests and just get going and not have to figure out federated search and subscriptions before they do (not as a replacement but in addition too). The added benefit is we could potentially build a database of what a server's network has access too and further help people figure out what server they want to join, so you get a little less dead Fediverse syndrome when you join a server that happens to block communities you would have been interested in. It of course could also be used to better refining searches in the first place.

Less feudal systems and more democracy for server admin, and community moderation I think will also help. Currently, admins and mods I think fall into lazie fair and organizers of the great purgers, it's almost always been this way to me too. I think this will help make more server more aligned to their user's interests and give servers a little more purpose for the end user.

More bridges! Matrix bridges (e.g. commune)! BlueSky bridges! Nostr bridges! Email bridges! SMS bridges! Signal bridges! XMPP bridges! IRC Bridges! More forum plugins and bridges! Q/A fediverse support! IndieWeb, just website bridges (good example bridgy-fed, but also the word press plugin! ). Meet people where they are. Make the Fediverse ubiquitous.

More selective federation rules, so you can have private server communities limit federation on per actor basis (Community/Group, User/Person, Post/Page, Comment/Note), maybe allow delays or rate limit federation, etc. Give servers and mods tools to be more granular on how they interact with the Fediverse so we get less ban hammer activity. This is most direct one to the current thread's debate, but I think we need to do more than defederate. I think more servers should have a limited federation policy with Threads because of it's size and influence, we want to interact with more people most of the time, but added where we need it and in ways mods and admins can handle (again more democratic systems could help here).

I also see a real potential for the fediverse everything app, but a big issue I see here is that the backend support is pretty tightly coupled with the fronts ends for most of the sites. At least there doesn't seem to be a lot of reusue for the server and interoperability with multi UIs. That seems like the first real step towards that.

Some of these are problems for devs to solve, some for admin to implement, some are documentation issues, some are just the people that need to know about them don't.