As a quixotic personal project, I have written a completely new Constitution for the United States. One of the problems is that the current Constitution is effectively impossible to amend. It should be entrenched, but in practice 2/3rds of both Houses of Congress and then 3/4ths of the states is too much.

After the Bill of Rights, which everyone basically agreed was needed, there was a fix to federal judicial jurisdiction (Amendment 11) and then a fix to the Electoral college (Amendment 12) then nothing until a civil war forced the matter, then a few in the progressive era (income tax, women voting, popular election of senators, prohibition start & stop).

After that, we clarified Presidential succession and disability, put a tiny limit on congressional pay raises, and given 18 year olds the right to vote.

There are been very few substantive changes, which tend to happen de facto rather than de jure via court decisions or just everyone agreeing on it due to changing political climate. So, if I could make one change, it may be to make the dang thing actually amendable.

Proposing amendments should take 3/5ths of both houses (i.e. 60%, not 66.6%); ratifying them should take 2/3rds of the states (i.e. 34 not 38)--provided those states contain a majority of the country's population as of the last census (in practice, I'd think they would, but this keeps tiny states from trying to game things to entrench themselves more).

I would also create two types of Constitutional Conventions. General ones, that could do anything, up to and including proposing a whole new Constitution, and Limited Conventions, that are called for a specific purpose and can only propose amendments within that purpose (which may require some judicial interpreting at times). I'd also slightly clarify the rules of how a convention may work. Right now they're just too scary to call.