Never playing one of these games with dialup, I would like to see an online multiplayer game were people can gather resources (wood, stones…) and build their houses or forts. The managers of the game would set up an world that would grow. A board game similar to this idea is Settlers. Where you try to take over the land with roads and buildings while working with the land around you.
The developers could have “spies” that are their own characters who add to the story by speading stories of treasures. They could cause all kinds of changes in the users life. I would give the user several vehicles to use. Air planes and underwater vehicles. I would not limit the user to land. I played a game called “Mean Street” that allowed side scrolling, 3D flying, and video conferencing.
Ideas for the world could be the Old West or Mad Max. I would consider adding character moves similar to Tomb Raiders.