A nationwide primary vote on a single day with 15% viability rule and 50% needed to win outright, followed by runoffs with greater thresholds to narrow the field. No delegates, no caucuses. The nomination can be secured in any one of these elections with 50%+1 of the votes. The convention may be sometime after in June/July/August. No more contested conventions, the nominee will be clear and undisputed.
I'm thinking something like this for the schedule:
March 3rd - first election - 15% viability
April 7th - second election - 25% viability
May 5th - third election - top-two
June 2nd - final election
I like it, but I hope the system doesn't need so make election days. I would worry about "voter weariness, or ability for folks to go vote so many times. I think at a minimum there should be two "Voting Holiday's" each year nationally, one for the Primaries and one in November.