Do you have a common pony fantasy? maybe with your favourite chosen mare.
I have this fantasy where Celestia sends out a decree; in order to keep the population alive, all stallions get to choose any two mares as their love slaves. Rainbow would be very against the idea at first, but she'd end up being the most dedicated to it, and in fact she'd be militant about it, since she's loyal and wants to carry out the Princesses' orders, and she'd find a whole new purpose living under a stallion as her master. This is my most standard thought toward my waifu slave Rainbow Sexy Dash. And I could command her to do anything, I would also choose Pinkie. I'd get Rainbow to bully and boss people around for me.