First skill I want to see that came to my mind would be a 3 min CD-Skill that would summon the opposite fairy you currently have for 10 seconds in addition to the one you have. She would sic-cast all her skills once (starting with silence/Magic resistance) and vanish.

Or even better, you gain a stack of fairy dust every 2 min. You can use a skill of a fairy currently not summoned. Only useable under the effect of fairy dust. Dissipation grants you a stack of fairy dust. (Wont happen due to the high requirements in hotbarspace and mikromanagement but I would like it.)