So here's what I'm thinking in a nutshell...
This is set in the pulp era, 1920s - 1940s, so assume that sort of technological and social levels of complexity and whatnot.
There is a small continent that exists in the gap between our dimension and another. You can only access this continent through a certain point on our world (we'll say Tierra Del Fuego or something just for the sake of argument for now). For security reasons, the existence of this continent is known only to a few people.
The continent is largely unexplored, mostly because exploration of the continent has been halted or slowed many times by the existence of a variety of beasts (dinosaurs and such) indigenous to the continent.
The United States and other interested parties have set up a 'base camp' just inside the entryway to this continent. It is from this 'base camp' that expeditions to the continent's interior are launched. The 'interested parties', also known as the Founders, intend to map out the continent and learn what exactly lives there (and also what sort of natural resources exist there) before informing the general public as to its existence.
That's where the Player Characters come in. They are explorers, sent into the unexplored interior of the continent on specific missions to learn more about the place. Since the Founders have a vague map of the exterior, and a general idea of the major landmarks (volcanos, mountains, vast forests, jungles, lakes can be seen from the air), missions often involve instructions such as "go to the base of the great volcano and detail what you find there".
Found on the continent will be not only dinosaurs, but also humanoids who have a very basic culture and language. There may be more than one group of said humanoids, and some of them may not look much like humans (they could be reptilian in appearance, for example). There will also be magic, the nature of which I haven't fully defined. I am not going to use Vancian magic, so I don't think I'll be able to use much of the extant Spellbound material unfortunately.
That's what I've got for now.