I suggest proceeding along the lines of the 8-part article I cited above, "Turning a Wife into a Hotwife."
If you don't like that approach, an alternative would be to propose a game in which your woman pretends to be your cuckoldress, and you pretend to be her cuckold. At first, make it a "just in bed" game, in which your woman seductively teases you about fucking a lover that day (she should give him a name, then expand until she has a stable of pretend lovers with different names), who was VERY satisfying.... his cock is MUCH larger than yours. Then, if you find that makes your time in bed together hotter, expand to her teasing you about her superior (pretend) lovers whenever the two of you are alone together, not necessarily in bed. That could gradually lead to her realizing that you are, in fact, turned on by thoughts of her fucking other men, which could lead to a specific agreement that it is totally OK for her to date, seduce, and fuck other men, while you will remain faithful as her cuckold (in reality).