Parallel replication does not account for multiple databases

In a situation where there are multiple databases on a single MariaDB server, a slave connected to the master will not apply statements from separate databases in parallel. Normally, a transaction will only affect a single database; even if it affects a large number of tables, all of those tables will still be in the same DB. Therefore, it seems like a singularly obvious optimization to parallel replication for updates to separate databases to be applied simultaneously when possible (with, if necessary, safeguards to ensure that transactions which do affect multiple databases block subsequent transactions to any of the affected databases so as not to apply updates out of order).