The way sports league subs magazines or whatever handle it is the greater league magazine, and then each team has their own team-specific place. It largely in effect already; you have the overarching "video games" sub and then specific games usually have their own sub for game-specific updates (here is what a vendor is selling today, build discussion, etc).

The issue with federated would be each instance is likely to have their own "parent" sub, with the specific ones probably falling to whatever instance has the established population.

It would be helpful, I think, for these "default" subs to have like a repository of sorts. Large topic subs all contribute to the same silo, and instances can pick what interactive content they get from the instances regarding comments and such. But that sorta defeats the de-federation tactics by link and article posts, but I imagine attack posts probably wouldn't fly in the vast majority of instances.