Sometimes you need a mechanic to do something. It doesn’t need to be a good mechanic, or at least that isn’t necessary yet. All you want is something that enables you to test another element of the design. In those cases, a strawman mechanic is what you’re looking for.
“Strawman” mechanics are an idea I got from Frank Lantz. They’re the dumbest, simplest things that do what you need done. Having strawmen in place makes it possible to test other aspects of the game; getting a high score may not be a great goal for your platformer, but it might be enough if you simply want to incentivize jumping so you can work out physics problems.
In addition, strawman mechanics provide a baseline against which other ideas can be judged. New mechanics have to work as well as, or preferably better than, the strawman. If they don’t, they don’t make the cut. This seemingly-trivial test will eliminate a lot of possibilities!
It’s often difficult to get all the aspects of a game right at once. You’ll find yourself needing to prop up one side of a game in order to work on the other. Let a strawman mechanic take that weight; it’ll keep your game in testing now, and help you choose a better system later.