The Best Man for the Job is a Woman
Setting: Not written in stone, but I'm thinking maybe sci-fi.
Location: Mobile, but also spending a lot of time on a two-person starship.
Characters: My male human and your futanari, though they don't have to be involved in smut together (see below).
This is all just brainstorming. The setting and details can easily be adjusted, but it's the theme that I'm interested in exploring. With that being said...
The idea involves our characters both working for some sort of galactic group of explorers, soldiers, etc. I imagine the galaxy being something more like Star Wars than Star Trek. Both characters get their new assignment aboard a two-person ship (similar to how the Millennium Falcon can mostly work with two people, I guess), where they'll be going on missions, exploring, and adventuring through the galaxy.
I envision my character as being quite good at what he does - all the things it takes to work for the people they work for. Having never met before, the two finally greet one another for the first time aboard the ship, both of the same rank and equal partners in their assignment. Considering it's sci-fi, I'm going to say they're wearing some sort of tight-fitting clothing or another. Regardless, this allows them to both notice something at the same time...
Your character has the bigger cock. Although neither says anything about it initially, my character is mortified by this discovery. Yours, however, is filled with silent glee and then takes charge immediately, pleased with her prevalence.
Note: The sizes here wouldn't be anything silly or unrealistic - so it's not like futanari all have ten-foot cocks or anything. They would fall within range of your typical human male, which is why she'd be pleased (and why it would be important to her) that hers was larger.
From there, the characters would get to know one another, but its quite clear who is in charge once the initial silent comparison is made. From there, as their adventures begin, my character eventually learns that yours is better than him at everything. Piloting, navigation, starship weaponry, handheld weapons, unarmed combat, electronics, etc. And any time he questions anything - or just because she feels like it - she reminds him who has the bigger cock (and it's why she's in charge), which brings her much delight to reiterate.
(And it's not that his is small and she's making fun of it; it's that hers is larger, which gives her great satisfaction.)
The RP: I see there being a lot of fun space adventures. There's quite a bit we could do with such a setting.
Theme: He's not a wimp or a sub; she's just better than him at everything and has a bigger cock, which she likes to remind him of.
The Smut: It doesn't even have to happen between our characters - at least not in the beginning. If it does happen, though, that's something I see as coming about gradually. In the meantime, however, I'd be more than happy to provide you with some temporary characters (NPCs) to enjoy as the story progresses. If you want female NPC type characters for your futa to pay with, no problem - I can supply tons of those, too!
If you're interested in discussing ideas, send me a PM! Please do not reply here. Thank you!