I think one thing people are going to get confused in what you're asking is the difference between making map variants a-la forge world and using Grognok to create areas in their entirety. I'm very doubtful we will get the tools to create areas using Grognok; it doesn't fit their goal of everything existing in one world if we can arbitrarily build levels. However I think PvP arenas with forge capability make perfect sense. You can combine custom games with forgeable levels into some sort of "training grounds." In the training grounds guardians would take advantage of a modular environment to prepare themselves for the challenges of the wild. This would fit better with the atmosphere Bungie is trying to create IMO.
However I wouldn't expect this at launch. I envision things like player economy and forge world to be mechanics to sweeten the pot of DLC content. The less moving pieces at launch the easy it will be to pull off a successful launch with the side benefit of having good mechanics to trickle into the game over time. Forge world would definitely be a reason for people to start playing Destiny again between major content releases.