As long as we’re tossing ideas out, here’s one of my (which to be clear is much less fundamentally important than the ones above, but could be a useful step if a proportional Senate is impossible):
Instead of requiring 60 Senate votes to invoke cloture, require votes representing 50% of the US population. Basically, a majority of the country can filibuster something that a minority is trying to push through the Senate.
Call it a “majority filibuster.”