I also had a very entertaining idea that I lack the skills to follow through with. In the same vein as "Changing Rooms"... "Changing Games."
Watch The Price rewind back past the Black Mesa Incident and decide not to put the crystal into the machine, while Gordon Freeman gets royally screwed by sand creatures in a Persian palace! See the Master Chief solving ancient conspiracies about the Knights Templar, while George and Nico refuse to leave their stasis pods aboard the Pillar of Autumn! Watch Lara run around collecting bananas in a giant plastic ball, while Monkey terrorises Albert back in Croft Manor!
It'd be hilarious, I tells ya. A little Flash movie or something. If anyone feels inspired.