An interface that would provide for the user to implement and pass back to the MongoDB Driver with methods that would get called on each type of event.
For example, instead of simply logging that a command is getting executed, you could call a method in the implemented interface (e.g. onCommandExecutionBegin) with the parameters that would otherwise be getting logged (e.g. command, connection, server).

This would give the user the ability to use their own logging mechanism and their own format.