I think defederating is easier said than done, and besides, what if one community is very well behaved and helpful and another is toxic and awful? You throw out the good with the bad in that case.
I think instead the user should be able to choose to combine similar communities, similar to the ‘multireddit’ concept. Then they can get lemmy.ml gaming and beehaw gaming in the same feed.
To help with discovery, a curated list could be created, and perhaps communities from that list could be suggested as time goes on. This does require some kind of centralisation but it would be down to the instance owner to decide to subscribe to it.
I would like to hear more about the move to Voat, what caused the failure in your opinion? I was not part of that as I had other things going on at the time.