Complete 5 Mission Arcs and...
How about the temp powers be purchasable through the system with Tickets?
This way players can buy the Temp Powers they want, instead of being issued ones for playing any type of missions.
...I think the point is to keep the regular, old content superior in some fashion to AE content, to encourage people to run it.
But that's just a theory.
If they did do this, I doubt they would make it based off of the number of missions that you do. It would make more sense for it to be simply purchased with AE tickets (and only usable in AE if possible).
This would put mission arcs even with radio missions. When you complete 3 radio missions you get a mayhem mission which gives useful temp powers. So what if AE missions also gave useful temp powers for completing 5 different missions of the same length.
5 short missions with custom groups - temp power
5 medium missions with custom groups - temp power
5 long missions with custom groups - temp power
5 very long missions with custom groups - temp power
5 short missions with default groups - temp power
5 medium missions with default groups - temp power
5 long missions with default groups - temp power
5 very long missions with default groups - temp power
The temp powers could be whatever creative ideas that would work in the system like Acrobatic Tumbling movement, Flying Carpet temp power, or a Golden Age Power Armor unlockable costume piece. The Devs could use whatever powers are just below the bubble in the massive list of power suggestions that have been made.