I'm not disagreeing with you on the point you're getting at, but no game engine from a studio that manages a franchise is truly "brand-new". It simply isn't feasible. You'll probably find CiV code in the Civ VI engine, nomatter how well-built it ends up being, or how fantastically it performs. I remember reading an article pointing out that Civ 1 code was still knocking around the codebase. Regardless of how it was made though, here's hoping for something that's definitely a new version compared to CiV.