If the Republicans want to avoid anoter McCain disaster, they need to modify their rules for primaries.
First, the winner take all system allowed on candidate with a minority of the total vote, and a very narrow plurality to take everything.
Second, McCain was able to win narrow pluralities in early primary states courtesy of the votes of non-Republicans in the Republican primaries. The Democrats have recognized the problem with such 'raidable' primaries, the Republicans should wake up to the problems, and close the primaries.
That was just the beginning of how rigged the Republican nomination system was. Winner-take-all primaries were concentrated in the North, while Southern primaries were mostly proportional. GWB stacked the deck as much as possible to nominate Rudy Giuliani. McCain merely seized the opportunity after Giuliani imploded.
Some of the mischief in the caucuses in West Virginia and Lousianna ["pro-life, pro-family"] are a whole nother subject.