@caifan,
I think you're asking the wrong questions. You ask if she could fall in love with someone else but, really, the question is, how did she fall out of love with you? "Out of the blue" she's told you she's unhappy? I'm thinking you've been asleep in your marriage for quite a few years and it's culminated to this.
Do you satisfy her? Obviously not. Whether that's emotionally, physically or financially, she wants more than what she has now. And that's not such a bad thing, progress is made because someone is unhappy about the way things are.
In the beginning when she mentioned going dancing, why did you act in disbelief? Did you actively want to go DANCING, as in, no kidding, actually dance with your wife? Did you offer to take dancing lessons? Or is it a ploy to meet someone at a club to have sex with? Ok, let's go with Option Three, to meet someone to have sex with.
Please explain how that is achievable when the spouse is glaring and obviously in discomfort over the situation. And please don't say either one one you would act with style and grace when you see your spouse walking out the club door with someone else.
So, on to an open marriage, you say. I don't think either one of you understand the concept of loving someone and having a private, separate sexual experience outside the marriage. It's Trust the binds you two together. Requiring her to "share EVERYTHING" is silly and not going to happen. It will make both of you start lying to each other, either to save face or refrain from unnecessarily hurting the other.
Please explain how you can now have "so much communication about everything" and yet still have lingering doubts? This is an open invitation to get into marriage counseling, if you both can articulate wants and needs as well as you claim, not an open marriage.
Or, maybe just admit this marriage isn't working for either of you.