I'm not a fan of tinkering with visas, personally, or leaving which students get to come up to Trump's DHS.
However, capping the % of foreign students at American universities wouldn't be the worst idea. You'd still have to let universities choose who to accept within those parameters
A number of state universities in a way view foreign students as a cash cow. Foreign students pay a lot in tuition, fees, and expenses and are often able to contribute more to the local economy than domestic students, given their financial situation. Losing foreign tuition money would require some budgetary re-working at many state universities.
Not that I'm saying it's a reason to forego thinking about a cap on foreign students, just that it's something that would need to be considered.