Yeah Dragon is saved either for Akainu of for Im or for Kong
Idk Dragon's role is a special one compared to the emperors and admirals. Dude's been building up his army in parallel to Luffy's rise since chapter 100. His army officers have bailed out luffy multiple times. Along with directly helped his crew grow strong.
Oda could either be setting up a Father & Son vs Imu fight.. or.. have Dragon fall to Imu for Luffy to then beat him.
Kong / Akainu dont make sense for me, due to not being CDs. Dragon's fight is directly with the CD, so he should help take down or atleast clash with the strongest of the CD.
His words to Ivankov were: "They'll watch as I change the world"
Only way to do that is to topple the King of the World.
Oda could also go with Garp/Dragon/Luffy , holy trinity type of a thing against IMU, since all 3 have something against the CD, Oda going out of his way to even emphasis in it for Garp in chapter 905 and then especially chapter 957.