I am working on an idea like this for a campaign I am putting together right now for a group of players I've played with for years (Wil, if you are reading this message, please stop now!!!)
I based my idea on the TV Show LOST. In the show LOST, There is essentially a group of people trapped on the island, but you constantly find new things to explore, new people come and go, entire groups also live there, etc. You eventually discover things like transporters, time travel, secret bases, etc.
My campaign doesn't take place on an island, it is a small village in the middle of nowhere, about 100 people living there. The party starts at first level and literally wanders into the village during a fierce and deadly thunderstorm, they were out looking for adventure and adventure they have found...
The basic idea is there is nothing specifically special about the village, but on the day after the storm, the party awakens in the small Inn to realize the village is under attack by some thugs. The party defends the peaceful farmers and village folks and become somewhat of local celebrities. They will eventually leave the village, thinking that is 'it', but after several days march north (or whatever direction they go in) they find out they are back in the village again.
No matter which direction they go, they end up back in the small village after only a few days travel. So, kind of like LOST, they are stuck in this one place and have to find a way out. Since there is travel in each direction, the 'closed world' they are stuck in will be large enough to add dungeons, small keeps, different groups of bad guys, ancient grave yards, a wizards tower, etc.
It will be determined that the village is not in the 'center' of the closed word, in fact there is going to be a small keep with a wizards tower that will be determined is the exact center. The keep is run down and falling apart and has bad guys and monsters in it, the Wizards tower is not accessible, protected by some sort of magic.
I want the campaign to have a lot of mystery and a lot of secret stuff going on, and every answer the PC's get will just open up more questions and mysteries...
The village will be the main 'base camp' of the PC's, since they are heroes in the village, they will have free room and food and discounts on what little supplies, weapons and armour the people there can offer. The town will be attacked from time to time and the party will protect it, but that will not be the focus of the campaign (although in the first session it will seem as though the objective was to simply defend the town from the attackers, etc.).
I've got more to do to finish the idea, but it is coming along pretty well. I've played a few good 4e games, but haven't had a nice and steady campaign since 3.5 a few years back.
D.