This is already somewhat possible if your instance support an OIDC solution, you could create one account and use it as login everywhere as long as it’s host by the same people.
For example I use authentik to create accounts and with OIDC my users can login to jellyfin, nextcloud, vaultwarden, forgejo, …
But that would mean putting all your eggs in one instance.
I don’t know if email is a good solution. Having contributed to the linux kernel ( in 2010ish ) may have distorted my perception of the gitmail workflow but it doesn’t seems very accesible and i can’t possibly see a change allowing this to be usable by semi-technical users.
We really need something like https://forgefed.org/ !
Running all our own forgejo instance is well and good but this the strengh of github is not in it’s feature but rather it’s because it offer the largest platform. Using a federation protocol would allow users to contribute without needing one account per instances.
Sadly github will continue to exist due to being where contributors are and being easy to find. I hope projects mirrors to forgejo as much as possible noneless.