I think this is definitely the right approach and would greatly help adoption. There’s so much potential for overlap under the current model (10 different communities for one sports team, for instance) and trying to just be a federated Reddit misses the mark IMO.
I took this approach with Magic: the Gathering at https://mtgzone.com which is just focused on MTG and has communities specific to the formats and interests areas within the game.
Great idea, I just created a Custom MTG community over at MTGZone: https://mtgzone.com/c/custom. Come join and get it going!