If it must be a Google Maps mashup, it should display house prices, electricity/water rates and rail delays on the appropriate pins. For bonus points, it should show the route from place to place, taking in the nearest water works etc. Creative interpretation of Chance and Community Chest is encouraged. No prize for putting community chest on a Soho strip club, OK? It's not that kind of chest..
Good luck finding Free Parking in London.
Update: the excellent Wayfaring has a London Monopoly Pub Crawl map.