The changes I would make to ODI's:
Get rid of the two new balls, because it takes reverse swing out of ODI's completely. The game is much more interesting when it's just one ball that reverses towards the end. The two new balls also causes more early batting collapses, which leads to one sided games.
Allow the powerplays to be taken at any time.
In the bowling powerplay, the fielding side should be allowed to have one extra fielder on field, so they have 12 men on the field. This gives the fielding side a genuine advantage during the bowling powerplay, whereas currently the fielding side are reluctant to take the bowling powerplay because it usually goes in favour of the batting side.
Be more lenient with wides down the legside.
Finally, all ODI series should be Tri-series. They are just more interesting.