Arma 3 comes to mind. Or Euro/American Truck Simulator.
That would indeed be amazing. Kind of a silly question, but having the world as a sandbox at your disposal has to be a game developer's dream and while it's technically very ambitious I have to wonder why nobody seems to be making any use of the fact that Outerra is trying to provide exactly that.
Simulating the world doesn't necessarily make a good game. People usually want simple rules in a simple environment and the ability to explore and stretch those rule as much as possible.
A random gigantic game that simulates absolutely everything doesn't let you do that.
Take KSP as an example, sure it would be pretty to use that engine, but that's it, it would just be prettier. It would'nt bring anything new. The game is already fun, not because it's realistic, but because it has much simpler orbit mechanics than reality. A model a regular human being can actually wrap his head around.
The most boring and disappointing part of KSP are the contracts. Why? Because they are random. They have no direction and no sensible progression. They don't encourage you to push the limits. They are just an endless generated generic grind.
I totally agree with your opinion of the default contract system. With mods it becomes WAY better. Contract Configurator [1] makes it really easy for people to make new contract packs with a simple config file and there are some pretty great packs out there that give it a much better progression.
Elite: Dangerous is would be close, although orbital mechanics are non-existent, the galaxy is modeled in the correct scale and now you can land on some planets.
The older Elite games (Frontier) actually had more correct orbital mechanics and space flight, and the creator didn't think it was very fun (dogfighting at km/s speeds etc.) so went back to a more arcade like model. I tend to agree, ED is pretty fun, although it has been described as "a mile wide and an inch deep".
Did you try Space Engineers? It has spaceship-building elements and also interactive planets (albeit tiny when compared to real planets, but still 120km diameter is enough to get a feeling you are on an actual planet). It also has multiplayer, although not in an MMO scale.
