my dream game is a survival horror RPG game where you could either stay and build up your stronghold or choose to roam from place to place trying to survive and searching for other survivors. Each would have their benefits and at any time you could choose to start making a new stronghold or leave your stronghold to roam.
The player would have to manage different things other than just staying alive, hunger, boredom, loneliness, they could get sick and die type of thing. The more the player would do one thing the better they would get at it, say constructing barricades or scavenging for supplies.
The game would have night and day cycles, there would be randomly spawned items and you could "craft" new items or make things that would have greater benefits.
And zombies of course.