That very question/idea has been considered for several years. Because women's golf has experienced a boon in recent years all over the world, specifically in South Korea, the concept of adding the "Rest of the World" as a third team to the Solheim Cup was debated. And there are tremendous reasons supporting such an event. However, due to the usual reasons (i.e. scheduling, agreement from all tours on all logistics, television, sponsorships, will it make money?), don't expect the Solheim Cup format to change. Personally, a round robin event in a three-team format would blow away the Solheim Cup in terms of excitement because of the level of talent not already in the event. That would threaten the tradition and perceived women's Ryder Cup feel. In other words, don't expect the LPGA and Euro LPGA to reach out for such a deal.
All that said, if it's going to happen, expect it soon. LPGA Commissioner Carolyn Bivens has been ahead of the curve on controversial issues like drug testing. Creating a three-team world event would be a feather in her cap and would leave a legacy more positive than the short trail she's already blazed. To make it work in the beginning, it would have to be held in the U.S. because the biggest television dollars are there, meaning that the event would need to run live when most fans are awake (as compared to this year's Solheim Cup in Sweden). Most of the best women's golfers are in the U.S. anyway so it's not impossible to visualize.
The moral of my answer is that NOW is the best time ever to ask your question. It's completely legitimate and timely.