Mine would be a FMF three way with one of the wife's hot friends. A couple of them are down for it, at least when we jokingly bring it up. Its just the whole dealing with it afterwords that is the big unknown.
An open/swinger relationship would also be something I would be interested in. Getting the wife to understand that fucking other people doesn't mean that I don't love her is the big obstacle here. At least with the 3some, we'd be doing it together.