The Problem
Real estate is becoming more of a do it yourself industry. With tools like Redfin and Zillow, it is increasingly easier to purchase a home without a buyers agent. There does not seem to be a seller agent killer yet, but it will happen eventually.
But the problem is, if you want to arrange a tour of open houses, you still need to open up a google maps window, estimate travel distances, estimate time at any given location, and figure out which open houses are open when.
The Business
Openhousemapmaker.com
You plug-in the houses you want to see, the site goes out and pulls the open house data and uses google maps to make your open house tour map. You simply open the app and drive.
Doableness
A good, but small programming team (or possibly a single programmer) could accomplish this task with relative ease. You would want to seek patent protection on it. (Make sure you add some new features, seeing as Estimated Future just made it obvious.) Without some form of protection, other real estate sites will simply copy you.
My Thoughts
One of the ways to monetize this concept is simply to build it and sell it to someone for whom it would be a nice feature. Not every business concept deserves to have an entire business ecosystem built around it. Build quickly, debug, then sell or license.