A multi-community would be all communities with a certain name, across all instances. It would prevent powermods from being a problem on Lemmy. i think it should be notated with m/<insert name here>, just like communities but with m instead of c.
Use case? Just wanting to group together certain topics. Grouping together all the duplicate communities that have the same focus as well.
Use case is obvious.
Making a ton of accounts for each topic is not a good solution.
You don’t have to make a ton of accounts. An account on one instance can subscribe to and participate in communities on any other instances (provided it hasn’t been defederated by the instance admin).