Don't exactly know the best way for this to be made. I'm guessing a special page built into the platform might actually be the best way. It'd just provide a list of all communities the instance federates with in order of ascending age.
Or, maybe more easily, an admin run community for people to post about their new communities?
I think so, it would be nice.
It might also help to discover early when someone has made a redundant community (one that already exists on another instance) and let them know and see if they want to reconsider.