I work on sharding at a bigcorp. Why not simply limit sign ups to a single Mastodon server which is then considered a shard in The overall fediverse. Stops one/some servers from becoming to powerful too
Also, while yes federated etc etc, I for sure get a lot more enjoyment from the "Local Timeline" in my server than trying to keep up with the absolute firehose of "Federated Timeline" which is another reason to pick one where the local content would be interesting