I would eliminate primary elections entirely, if I could. They have failed, again and again, to either identify or choose the best candidate.
If I could not eliminate them, I would limit the franchise to enrolled party members who had been enrolled continuously since the last Presidential election, and who could prove that they had donated either >$5000 or had given at least 625 volunteer hours to party organizations since the last Presidential election.
If I could do neither (and a lot of states have passed laws governing primaries), I would make sure that any delegate selected this way had no right to vote at the National Convention, which should be held no earlier than September 25-30 of the year of the election.
2) Change the bylaws or structure of the Republican party so that we can throw the venomous traitorous louses out.
3) Basic campaign finance laws. You can't donate to anyone you can't vote for. That includes corporation, unions, trusts, whatever. If it's an entity that can't vote it can't donate.
4)We need to take back our party. It has been co-opted by liberals and we need to do it at the grass root, town and state level before we worry about national politics.
These changes would only stop the bleeding.