Being able to migrate an account between instances would help a huge amount. I am almost wishing we could agree on a default instance for all newcomers, but this would work to just centralize the user base so it's not a good solution. However, if account migration were implemented, there could be a sort of "first steps" or "training ground" instance that accepts all new users by default and only allows accounts to remain there for a set period of time before asking them to find a new instance to call home. This would let users who aren't interested in the technical details of federation get their feet wet and figure out which instance aligns with them before needing to make the choice.

This scheme would definitely require that new users have the option of signing up to a particular instance right away; no need for the default instance to be the only sign up option.