I'm a bit late in bringing this article up but I thought I'd point it out as an interesting approach to resolving the difficulties in making a game fun in both single-player and multi-player modes in different ways.  It's also a pretty good example of thinking well out of the box.  I don't expect Stardock has the resources to (essentially) create 2 separate games, but it's an approach that we could pursue in modding. 

http://arstechnica.com/gaming/news/2010/04/starcraft-2.ars