Instead of income based repayment, I would rather see very low interest rates on student loans (like 1.5%). That way everyone pays the full amount they borrow, there is still incentive to repay promptly, borrow less, earn a higher wage, and those with low income can at least pay off the loan interest so their loan does not balloon. And it's simpler.

Another thought I had (not completely fleshed out)... Federal student loans could help cap tuition costs somewhat like medicaid/medicare. The fed offers a fixed amount per semester (say $4k) to each student. The catch is that a uni can only accept fed loans if their tuition is fixed at the cap. (i believe this is how medicare negotiates reasonable prices for meds - "if you want to accept medicare here is what we will pay for Rx, and it's all or nothing").