Sorry, I meant if you had to make a new game in one of the genre's that you've already made games in.
I think you're right about any game needing lots of Physics being naturally difficult, like if you had to make a version of Kerbal Space Program or something.
Having played around with both, I'd have to say that turn-based games are "easier" simply because the game "pauses" at regular intervals, making it much easier to evaluate different things. In a real-time game, I frequently have to press the Pause button to evaluate the coordinates of sprites or see if damage is being calculated correctly (or at all).
I have another suspicion that 2D games are easier to make than 3D games . . . but I can't say for sure because I haven't made anything in 3D yet.