For me, an open marriage has been about exploring what has seemed natural but was not "allowed" in a monogamous marriage,) not forcing something out of some sort of desperation. The people who would make good partners in an open relationship are the same people who might make good friends or lovers in a closed one, with one caveat – they necessarily don't have to "have it all."
It doesn't matter if he doesn't want kids and you've always imagined having a big family. It makes no difference if she wants to live in the country and you've always been the city type. And it won't be an issue if he's a homebody and you like to always be on the go. Mutual desire, attraction, and a clear understanding of the boundaries are all you need.
And perhaps that is the most satisfying element of all when it comes to open relationships – having the opportunity to experience people fully.