The popular belief today is that Disney won't be building a fifth park anytime soon. Rather, they'll make significant additions to existing parks over the course of the next 3 years(ie-Beastly Kingdom at the Animal Kingdom, and new attractions at the other parks).
I doubt that they will expand the monorail lines. It's really really expensive (1 mill per mile) and I doubt disney would spend 1 billion on monorails. I hear they're looking into building a light-rail network.
And don't look for a 5th park anytime soon. They're probably going to work on building up the 4 existing parks. The attendance figures don't warrant a 5th park (even before the tragedy).