OK, I'll say right out that I'm just a baby, learning, here. I know - or think I know - a few things about Diocletian. Let me sloppily throw them out for conversation, because in my own mind, this guy was a symbol about everything that was great, and everything that was terrible, about the later Empire:

1. The Succession Thing. Obviously, after a horrendous 50 years of chaos with one Soldier-Emperor after another murdering his rivals, claiming the throne, screwing it up, and being murdered after another, the Succession Thing had to be fixed. Since it was the essence of human nature in the struggle for power, I think it was very realistic of Diocletian to create not one, but FOUR positions, of influence. One Emperor for East, one for West. Each with a Caesar under him in a support and/or learning position, to also serve in their turn as Emperors of East and West (with two new Caesars coming up in their places).

It's logical, and it didn't last even as far as Constantine; literally when Diocletian retired, the 'chosen' all fell to fighting and murdering each other, just like before. What DID survive was the notion of a split between the Eastern Empire and the Western Empire, as if they were no longer one, but essentially different; and that, perhaps, they were even in opposition to each other's interests.

2. The Army Thing. It is my impression that Diocletian was the first Emperor to vastly increase the size of the armies and to impose those taxes to pay for them that would become such an incredible burden upon the generations to come. I remember reading a book of (conventional wisdom) history by Michael Grant ages ago, in which he very firmly argued that one of the reasons the Romans didn't fight harder against the invading barbarians was that they were in a position where all economic initiative had been simply crushed by the taxes required to pay for the army. (Of course, if they HADN'T had the taxes to pay for the army, we would have had the Fall of the West a century earlier).

3. The whole "The Emperor is a Divinity" Thing. Of course, if you've had 50 years of murderous power struggles, Diocletian's idea to raise up the role of the Emperor with very eastern-type semi-religious awe, into a position where court protocols were endless and intimidating, to essentially de-humanize the rulers of the Empire and hedge them about with all sorts of divine BS to awe the uninitiated, may make sense. But it's precisely that kind of eastern garbage that makes me wince in the last century of the Emperors, and on into Byzantium. Was it something that worked?

You might say my ideas about Diocletian are very confused, because in all these changes he did something that perhaps was very necessary - but something that I'm not sure was sensible in the long run. But whether doing nothing would have been better?....