To be fair, to do that he had to institute the Tetrarchy - essentially accepting that the Roman empire was far too big to be effectively governed by one guy, and splitting it. And while the Eastern Empire indeed survived for long, it lost a lot of land and had to adopt a more defensive, "let's hold onto what we've got" approach rather than seeking aggressive expansion - after Justinian, they were mostly either losing territory or barely defending what they had, with occasional exceptions like the Komnenian restoration and Basil II having them recover sizeable portions of land.

So, going by this analogy, the US would need to split up in several states, and really cut down on overseas imperialism to focus on maintaining what they have (which would be pretty good for the rest of us). The impressive thing about Diocletian's system is that it didn't immediately lead to the Eastern half saying "yeah, fuck you guys, we're doing our own thing now" - it wasn't a Balkanization, since the new split states weren't hostile to one another. I'm not sure if that could really happen with America.

It would be really funny to see the tradition of junior emperors being brought to the US however - just imagine failsons like Hunter and the Trump kids being deputized to rule the West Coast, it'd be great