Interesting idea I read about a while back:
You set up two boards, say monopoly and risk, set a square and path on each of the boards to be a gateway. Let's say the railroads and the corner countries. You use whatever characters you want, and those characters are your piece in monopoly, and your champion in risk. when crossing from monopoly to risk, money can be traded for troops by $100 = 1 troop, and likewise from risk to monopoly. If a person's commander dies in risk, or they go bankrupt in monopoly, they lose from both games. The gateways are used to freely travel through the different boards, and when travelling from one to another, the players can bring money or troops along with them which will be converted.