[lordpidey]

An interesting solution would be that EVERY (non-base) class has a skill that can ONLY be used while monoclassing that plays into that class's gimmick.

Let firebird feathers resonate 3x.
Let kensai skills activate as though they were comboed 50% of the time.
Reduce Ki usage by 25%, without decreasing the power. (So even Kahouda leaves behind 25% ki for example)
Evokers... Hmm... Let them start the fight with an invocation half finished.

I'm sure we can think of more.