The architecture of the WiredTiger storage engine implementation means that MongoDB maintains two copies of data to guarantee durability, one in the oplog the other in the WiredTiger log (journal). It is only necessary to ensure that the content of the oplog is made durable to the current guaranteed durability point, since the oplog can be used to "recover" the content in other collections after a crash.
This work encompasses switching which content is journaled when using the WiredTiger storage engine.
There are cases where the storage engine will need to keep journaling as it does today - in any configuration where the oplog isn't enabled, so it makes sense to add this feature alongside the current method of journaling. Perhaps we need a new (or expanded) parameter to the KVEngine constructor to decide which journaling mode to use.