As long as sub Riya is the slow burn kind (Takes awhile to do) then I think i have an idea.
Riya doesnt seem like someone who would back down from a challenge, so the PC simply offers her one (A bit like Syri's bar scenes). I mean she
clearly superior to a filthy alien/ rich kid right? Have something on the line, perhaps someone being on top? or gets some ass for day from the loser?
If Riya loses she'll sure be upset, but a deal is a deal so she goes along with what you and her betted. After some hot dom stuff for whoever the victor is, lets go with Steele, later on another bet could be placed as Riya would want to get back at you.
Win enough times and she eventually likes the after-play, succumbing to whatever depravity you do to her. It culminates to her being your sub. She can keep the same attitude to keep up appearances with everyone else, but we all know what she really wants
.
This, essentially, is almost identical to how Izma from CoC worked, which I thought was done VERY well. Especially because it could be a double edged sword. Lose enough times to Izma and you become the sub to your new Alpha. The same idea could apply to Riya.
It could be a test of combat, strength, or Video Games (After all she likes playing them). Maybe tweaking this system with your own twist could work out better. Maybe some subtle manipulation to quicken the process, or some dommy sex stuff like slave Sera.
If you were gonna do this, I wouldnt make the option of making her a follower like Izma due to it being more than a hassle. Also for Nooby, I wouldnt de-dick her. That may cause to much sadness for the poor bastard.
This is me just throwing my hat into the ring. Whatever you wind up doing, I know it will be good and I look forward to seeing it.