how about allowing the player to buy from the c-store outfits from a different faction then their own.
And with that i mean piece by piece, not entire outfits, since i guess most would want only parts of them.
That way you can also have a RP reason for the piece, war trophies, gifts from someone you saved, family heirloom and so on.
Followed by a strange idea, make the prices for the pieces depending on how many you already have, getting more and more expensive the more pieces the player buys to keep it in the spirit of the idea.
And i would make them per character as well.