Hehe, yes! I actually had exactly that in mind as well: Koopas that you have to line up to take out a bunch of Goombas, and you have to plan it so that Jumping Man steps on the Koopa at the right moment. It was the natural next thing to make, just not enough time for that mechanic. :)
I also was planning pipes that you could use to warp your creatures into position, including some that would be in otherwise unreachable locations.
And I was planning hazards that could kill Jumping Man as well (just like a regular Mario level) and you would have to help manipulate his movement so that he avoids them. 🙃