Working with very large phpbb installations becomes difficult with just a single database. Although there are tools for spreading out the load they are difficult to setup or don't provide a complete solution.
Ideally, phpbb should support writing to a master DSN, and reading from another. This can be achieved by detecting the query type (eg. looking for insert,update,delete,..) anywhere in the SQL and using the master connection for that query.
This feature should allow for the addition of an unlimited number of masters and unlimited number of slaves. If more than one type is available for each then they should be selected at random (more complex resolutions can be added later).