The first primary should be held in the state that had the greatest percentage of registered voters vote in the previous presidential election.
It's simple. The 2nd primary would go to the second place state and so on. It would be a competition for each state to GOTV - a boon for democracy and a competition amongst the states for voters and parties. This would also take away those pesky problems of which state goes first. After each presidential election the first primary state for the next presidential election would be known after the votes are all counted.
Caucus states would have an incentive to either increase participation or change to a primary.
It basically rewards the states’ gotv operation!