While doing some research on mega dungeons I've come across a bunch of interesting blog posts and resources. Some of which have given be quite a lot to think about. In this post by the Greyhawk Gronard http://greyhawkgrognard.blogspot.it/2008/12/old-school-campaign-tent-poles.html he points out the following:
I happen to agree whole-heartedly, but it's important to remember (as James has) that just because a campaign has a central megadungeon as its focus, it is by no means the exclusive province of the campaign. Adventurers are free to delve into its mouldy depths if they choose, but they're just as free to poke around in the local town or city, hie off into the wilderness, or explore smaller dungeons that the DM should properly have waiting for those eager for a change of pace.Which leads me to believe, that I may just go ahead and combine the mega dungeon with a little campaign setting I was working on. Which of course will mean quite a lot more writing, which I'm fine with.