Every once in a while I'll get it in my head to make a 3D game, rather than a 2D one. But I don't want to spend the time building any assets - not only am I bad at that, but I also don't know if my idea is good until people try it and give me feedback... which requires a working prototype.

What I'm looking for is a game engine (asset library?) that lets me build my town by placing buildings, and dropping in my main character - and then I can just run around the town, opening doors and talking to people. Tons of tons of game have dialogue, medieval fighting, assassinations, kill moves - I don't want to have to spend time making any of that. I want to test my game ideas and see if they are fun. I want to write the parts that are unique to my game, the story and the decisions you make. Then, later, I can go back and iterate on assets.

Both game engines now have Asset stores, but I'm not sure the level designer is to the point where I can quickly iterate over my ideas.