If I make a game where players can build anything they want, will I be responsible for any inappropriate content?
In my opinion, I think it could be the player that is responsible, if he/she doesn’t follow the rules or guidelines, maybe I am wrong or right, never had a issue of that.
You should not be held responsible, as you are not in charge of something that another peabrain thinks of.
On the other hand, the game idea might not be the best of ideas, considering you are worried about this in the first place. If you would still go along with the idea, I would suggest just giving the player basic builds. For example, something they will not take advantage of and design something against ToS.