I would be fine with Iowa getting bumped after Nevada and South Carolina given their massive clusterf**k caucus this year.
If we're keeping roughly the current system intact, it's probably the path of least resistance and easiest change to make to bump Iowa, ban caucuses, and make the list New Hampshire/Nevada (a primary)/South Carolina/New state. I've heard good arguments for Illinois as the new state for #4...a big state, yes, but a big state going fourth isn't the same as it going first. Another possibility might include Michigan. A big northern state in the Midwest.
EDIT: another advantage of this might be that it'd be a good calendar for the GOP as well, and the two parties like to keep their two calendars more or less harmonious. NH/NV/SC/IL works fine as a first four for the GOP and they no longer have to worry about the Iowa caucus giving a huge bump to random religious charlatans.