Github's huge win is the network effect of having one account and being able to interact on thousands of issue trackers, create PRs, etc. It would super cool if GitLab could achieve some of that same experience without requiring everyone to all be on gitlab.com. It would go a long way toward addressing the criticisms that Github is too centralized.
Yeah, I think it'd be awesome if there was some form of optional federation between GitLab instances (sort of like Matrix or NextCloud).
I also recommend to check out Gitea (community fork of Gogs) if you haven't yet, it's a nice alternative to Gitlab for selfhosting.