Imo, there should be a single national primary. But beyond that, I think that trying to find a good way to change system, while keeping the staggered calendar would be tough. There's no obvious replacement for Iowa that I can see being broadly agreed on. It looks more likely that Nevada will be replaced in 2020 with Colorado or another Western state, at least on the GOP side, if they're a mess again.
I think there's a fair bit of value in the staggered system. It allows more opportunities for lesser funded and lesser known candidates a better chance of winning.