I remember the missions in CoH that had some unique temp powers, vaguely. But this could be a tool of great use for player-generated missions.
A simple menu is created to hold info like # of uses, name, description, etc.. A pool of effects are available, as well as dummy effects, and a pool of all animations of other powers is also available, maybe plus a few more.

Dummy effects could be things like "Self: antidote", "Ranged killswitch", "Melee: Kryptonite Injection", etc. And dummy recipients could be made similarly.
-On a critter, a dummy power "Kryptonite Weakness" could be made and the dummy effect "Kryptonite Damage" could be slotted under the dummy power.
-Glowies could have a slot to receive dummy effects similarly.

Here's a few examples of how this could turn out:
-In a mission of "Get poison antidote to Rott before it's too late" (timer here), you have to fight through mobs of baddies to get to Rott fighting a super soldier. Rott has already been poisoned but it hasn't taken effect, meanwhile, he's toe-to-toe with the Boss, if the timer runs to, say, 30 min out of a full hour, his resistances decrease and the boss starts to damage him. You or your team either swoop in and try to defeat the Boss with Rott, but the boss won't let up, or you can use the Give Antidote temp power on Rott and he and you (or your team) defeat this boss together.
-Similar, the mayor is poisoned, but it's latent, he's giving a speech in Phoenix Plaza, you have, say, 30 mins to get the antidote to him or [insert bad thing].
-EdenTech creates a monster that rampages through Ironport, it's nearly indestructible due to it's high regen rate (Imagine Moment of Glory auto), but your contact gives you a "Ethyl Chloride Dart" temp power with the dummy effect "EtCl damage". The monster has the dummy auto power "EtCl weakness" which has the effect of lowering his regen rate by 90%, making him far more vincible.
-Doctor Tyche empowers himself with the power of Magnum Magus, an ancient wizard, and can use blasts of negative energy to defeat foes. Lucky for you, Magus's lieutenant has given you the secrets to him amulet which he uses to protect himself from Magus's negative energy attacks, and your tech team has converted it into a shield; however, this shield only has 5 minutes of charge (the temp power has a dummy effect "Neg Damage - 50%) and Tyche's attacks have "Negative Damage..." (unless that becomes a core damage type)). Tyche would be very difficult (or impossible) to defeat without this shield on during his attacks.
...and many more.

Maybe these powers could be kept when the Arc is over, but if it's easier to implement, they disappear upon Arc completion.

I think this would be a great addition to such a mission creating system, and brings us closer to the devs' missions with this tool in our creative arsenal.

That being said, I wouldn't expect such a thing at launch, but infrastructure shouldn't impede something like this.

Thoughts?