Establishing a unicameral legislature elected via STV, with each state as a multi-member constituency and seats allocated between states according to the Wyoming Rule; Electoral College abolition and replacement with the nationwide popular vote; term limits for federal judges; hard caps on corporate donations to campaigns.
My grand political ideals aren't really suitable for an amendment, so this is what I'd go for.
This is all good stuff, except for the Wyoming rule, which could lead to a massively over- or undersized legislature. Much better to stick to a fixed number - maybe 700-800?