Hm, then I feel like students might do the minimum passable work. Which, to be fair, is pretty close to optimal for the real world.
Maybe that's the trick - make the requirements for passing good enough that the minimum still requires learning and engagement, and then just grade on a straight pass/fail.
That's how a bunch of courses I attended and TAed did it. You needed to beat a specific rating averaged over your homework assignments to be allowed to take the exams. Occasionally caused really good students to fluke the last 1 or 2 assignments since they were sure to have qualified, but generally kept everyone doing them and allowed for failing occasionally (which is useful because students then are less motivated to try and hide their failure, which gives the TAs better feedback about progress and problems)
Maybe that's the trick - make the requirements for passing good enough that the minimum still requires learning and engagement, and then just grade on a straight pass/fail.