Well this is not the first time it has been asked/requested, every time Ashley has answered that he preferred to keep focused on C2 until completion.
A 3D engine is a LOT of work, and wouldn't provide the same accessibility than C2 anyway, since 3D deals with more complex maths/concepts than 2D.
That is what would make this so great,If Ashley could find a way to make it less complex for people like me,Who by the way loves unity but will never grasp 3D elements of code and how it works.The same learning aspects as C2 would carry over to possibly C3D,But i do understand what you mean by more complex maths and concepts.
I still think Ashley and the Scirra Team could make an Incredible dream a reality for alot of people who would love to make 3D games.Even at a miniscule scale.