Common database shared to other database
It is not a bad design at all, and it is not something new.

In MySQL any operation involving multiple tables can also involve multiple databases, so you won't have functional limitations.

The drawback is the complexity that you will need to add if you need to shard the databases across multiple servers/clusters. Because you will need a way to replicate the common_db, and nothing else, to all servers. But, even if you choice another model, you will hardly have an easy solution for this problem.