Here James Mishler relates the account of how the Wilderlands of High Fantasy started as a successor to a campaign in Middle Earth that Bob Bledsaw was running. The characters hopped in a portal and found themselves in a strange new world where they could explore and make their mark.
Ironically that nearly describes how my first Wilderlands campaign started. I was running a small gaming group whose characters were known as the Reign. The campaign world was the World of Greyhawk.
By the time they reached 10th level or so I was growing dissatsified with running Greyhawk. The howling emptiness of the 30 mile hexes were tedious to fill. The various realms were too grand. Their scale was beyond what I felt even 10th level characters could measurably effect. Finally I just needed more support due to my relative inexperience at the time.
The Wilderlands freed me from generating the low level details and allowed me to focus on the grand overview. It's scale was near prefect as players would definitely have an impact on the various town and villages described. So one session I annouced that I was moving the campaign over to the Wilderlands. A portal appeared before the Reign, yes I was that heavy handed, and they could go through or be left behind.
When they went through the port they found themselves in the Wilderlands and began exploring. By the time they reached 15th to 16 th level they were well on their way on estabilshing realms of their own. The earliest notes I still have date from this campaign.