I think XYZ123 brings up a good point. You may want to decide which route do you want to go down, the more poly route or the swinging route. (There is a lot of cross over, but if you want to talk to your wife, you probably just want to talk about which one you want to focus on.) For example, my wife has a hard time accepting polyamory. She is much more open to swinging though since that is not devaluing the relationship. (Her perspective is that if I love someone else romantically, then our relationship is no longer unique to us and therefore less valuable.)
If you want to go the polyamorous route, you should probably take it slow and let her get use to it as it progresses. Let her know there will be time to adjust and her feelings will not be ignored.
But that is after you first bring it up.
You could bring up the concept by asking her how she would if she had met two of her past lovers at the same time. You could try to get her to read a Heinlein book if she likes sci-fi and talk about if afterwards to see what she thought of the relationships in it. Or you could just come uot and just say, "Honey, there is something that has been on my mind." And rely on being able to handle questions from the logical to the emotional.
I wish you luck.