Alrighty, so just from a recent game I've been playing, I've gotten it in my head that it might be fun to play a crossdressing RP between two people that are in a relationship.
First, the woman dresses essentially as a male, wearing suits and other masculine clothing, though whether she takes it farther than that is up to you.
She's also clearly the dominant one in the relationship.
Second, The male character dresses in feminine clothing (Dresses, etc.) and is a bit petite and feminine looking enough to pull it off quite well, much to the delight of his girlfriend, who enjoys dressing him as such, even though he may be a bit embarrassed by it.
As for the rest, we can come up with the details together, but that's the main idea...lol