I've said this before: it sounds irksome to me when people speak of "opening up their marriage" or "adding another person to their relationship". It's like saying "here's the deal and you can join too". It sounds like you have a sports-team and you're looking for someone to play a particular position.
My husband and I are gradually hashing out the theory-versus-reality of having other intimate/romantic relationships besides our own and it in no way feels like we're "changing" our marriage (we got married for pragmatic reasons, although we do have sex and affection). If something is "missing" in one relationship, I don't think the answer is to try to fill the void with another relationship. I think it's about meeting a person or persons that you would like to have a relationship with and trying to fit that into your life. I don't go around thinking in terms of "my husband doesn't like some of the same hobbies as i do so i think i need a relationship with someone who likes X, Y, and Z." But, I am a bit of a misanthrope and don't make a point to meet people just for the sake of meeting people. I find more quality over quantity by just doing what interests me and if I make new friends, then that's like extra gravy. That was how i met S in the first place.