It's hard to settle on any one specific dream game, but if I were to give one idea, my dream game would be a sort of tactical and social hybrid game in either a post-apocalyptic setting or an off-world colony on a relatively desolate world with a few settlements. It would not play like classic dungeons and dragons inspired role-playing games, but a combination of a group-based tactical game with a more freestyle Sims-like toybox/dollhouse simulation, where you can control multiple characters in a gang or family of sorts that move around together and can be productive and/or fight stuff.
Combat would take inspiration from the Games Workshop tabletop game Gorkamorka, in which you would control a gang of Orks as they rode around the desert on trucks and bikes collecting scrap and fighting, gaining experience as they raced around such. The exact implementation I would like to see would be more like the game X-Com's tactical combat, but having a "team" or "gang" of distinct recruited characters which ride around together and have adventures is an important aspect of the idea, and to fight scavenger wars in the wilderness with rival gangs or hostile locals fits the feel I'm going for.
Individual members of the gang have their own skill sets and abilities, no rigid character classes, just general specializations by the skills they practice -- doctor, mechanic, brute, things like that. Beyond that, they'd have personality and relationships with each other and outside the gang, similar to a household in The Sims. The television show Firefly has a group of diverse characters riding around in a beat up old ship having adventures, and gives an idea of the feel I'm going for here.
In addition to combat and personal dynamics, vehicles, a ship, or some other kind of mechanical "stuff" to manage and serve as a mobile home of sorts in your squad's adventures is important, complete with customizations, strap-on weapons, etc., and a home base of some sort, whether it be a cave or bunker or some kind of surface structure, even a building in a city. Regardless of what it is, to be customizable and designable relatively freely would be important, with inspirations here both from The Sims and Dwarf Fortress.
Ideally, I would like it to be such that fighting and acting like a gang or fighting unit rather than just a family or group of friends would be completely optional and unnecessary, with plenty of opportunities to either work on self-sufficiency by scratching out survival in the wilderness, similar to Dwarf Fortress, or to participate in civilization more directly by passing your trades as doctor, mechanic, and what have you, like the Sims.
Eventually, to be able to have a sort of permanent wilderness settlement grow into a small town like in Dwarf Fortress would be nice, whether because others are settling right nearby and working with your people as NPCs, or because your group grows bigger and bigger until you spend little time micromanaging them. The Sims would do all of this by simply letting you start a new family and having them move next to your old family and build a home next door. That kind of player-dominated development, with the computer maintaining your other characters in the same world when you aren't using them, is probably what I would settle on.
Essentially, it would have the world building of Dwarf Fortress, small scale and character interactions of The Sims, and the combat and squad management of X-Com, wrapped in a sort of space western or post-apocalyptic setting.