You know the drill, 5 votes and said thing gets added in! (A) Keep the 2D of BTD as it is now. (B) Switch to 2.5D.
Know that BTD will switch to 2.5D in 0.6, just putting this now because BTD might cease development soon. Also here's a little thing on what the 2.5D would look like: https://scratch.mit.edu/projects/960811446/ And C2A.