Also, are you sure that your wife can't/won't bottom at all? Have you talked with her about it? Maybe there is a level of play or type of play that she could accept, even if it is not her preference, that could give you what you want/need, too? No, I am not saying you should pressure her into anything, or that she do anything she truly finds distasteful, as that would be unfair to both of you in the long run. But I sometimes do or try things that aren't on my top ten list that my partner really enjoys. Sometimes, I just put up with it (my thing, not saying it is right for anyone else, and NOT things I am repulsed by or hate, just things I don't love,) sometimes, I have found that I do enjoy them once I try them. I might also try to get my partner on this site, let them see what the BDSM community and lifestyle really is. Maybe if it is a little less scary, she might find some interest.
I'm putting this out there because, if you met me in r/l, you would probably never know just how submissive I can be in private, with the right person. The way you phrased your comment about your wife makes me think that she is very strong in real life--but a lot of subs are, and a lot of tops have much less power in everyday life than they do in their sex lives.
I, too, am in a semi-open marriage (I only have one other partner,) because I wasn't getting what I needed from my husband, but we have an 8yr-old and just had our 11th anniversary this Friday, so we don't want to split. My husband does not have another partner yet, but he knows that he can. If you do decide, together, to go open with yours, make sure you both understand exactly what that means-can you have one lover? many? does she/do you want to know about who you/she is with? are your lovers allowed in the house, or do you need to meet elsewhere?
I am not trying to tell you what to do, by any means. It's just that I have seen open relationships work very well, and I have seen people get really hurt. Since this open-ness is very new for me, the issues are fresh in my mind.
Best of luck to both of you!