The closest thing to what you are looking for is BigWorld
Huh. I honestly had no idea that Wargaming still sold new licenses to third parties.
That said, BW is definitely not what the OP is asking for. Right from their own FAQ (emphasis added):
How much programming experience do I need? Is it a drag and drop / WYSIWYG application?
- You need knowledge of Linux and Windows game development ideally. This is not a drag and drop game kit.
- You will require knowledge of Python to implement your game ideas and drive the machinery of the game logic.
You could also look at the open source Ryzom Core, which has proven capabilities
Are the Ryzom Core tools any good? If nothing else, looking through the public code... it's very, very game-specific in places (e.g., I see hard-coded assumptions about the game's races throughout the shared and tools code.)
Also, it's all Affero GPL (so less "Open Source" and more RMS-style "Free Software"), so the project is basically useless for anyone who plans to actually invest in making a game rather than just doing it as a hobby.