Dude. She's already brought it up, and from the sound of things, she's probably already the dominant one... you're probably already in the submissive position posing as otherwise. A LOT of women are naturally dominant and fine with having submissive significants. The idea that ALL women ONLY want mr masculinity is a misnomer that I can dispel for you immediately.
I remember when I was in my early 20's, my fiance brought up the idea of using a strap-on and I shrugged it off even though it turned me on. She wasn't the 'right' one for me, so it wouldn't have worked out anyway, but I regret that I wasn't able to be open and honest with her about my true feelings. My wife fucks me at least once a month. I enjoy it very much and am working towards being able to cum hands free. The harness I bought us has inserts for to enjoy the feeling, plus she gets a psychological thrill from fucking me. I think she likes the power-play and role reversal, but I know she also loves doing things that make me happy. So, I enjoy her fucking me because she enjoys fucking me because I enjoy her fucking me... it's a bit of the chicken or the egg scenario, but the point I'm trying to make is we have a VERY healthy relationship even though I take the submissive role -- especially in bed.
Healthy communication is the foundation of all good relationships, so simply have a conversation. Maybe you should both have a couple of takes before you bring it up (not suggesting you go get hammered and then try talking this one out either). If I were you, I'd start by prefacing what you're about to say with the fact that you feel nervous and a little insecure about what you're about to say (so she can be sure to respond appropriately). Then, remind her of 'that time' in the sex toy shop when she... you get the point.
You might be nervous to talk about this side of your sex life, but you shouldn't be. If you're in the relationship you're meant to be in, you should DEFINITELY be able to bring this up. I think if it were possible to have the actual stats on relationships where the man is in the submissive role, you'd be shocked.
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