The issue from the beginning was that people wanted to pick where there were already discussions. .world had already started growing a lot more, so new users gravitated to that. As drama happened, there was some splintering, but since you can’t take your history and account with you, there was also resistance to move. What you suggest was discussed during the first big migration to the Fediverse for Lemmy, but it takes time to put things in place and it was already too late to really fix the lopsidedness of who was where.

It’s still a good idea for new users, maybe instead of randomizing have a way to categorize the types of instances and their rules, so people go where they’ll be the happiest. The best way to fix the problem though is to come up with a secure and privacy protecting way to fully move a user from instance to instance. Good luck with that.