The Emperor prior to Diocletian was Numerian (r. 282 - 284 AD).
He ruled the eastern part of the Empire.
Yes, that is right, he was not the only Emperor at the time.
There was another, ruling the western part of the Empire, Numerian's brother, Carinus (r. 282 - 285 AD).
So we see in place a system of divided, ostensibly harmonious rule, by two Emperors, due to the enormity of managing the frontiers.
Yet this system was done in the reign of Valerian ( r. 253 - 260 AD) who took responsibility of the eastern part of the Empire and made his son, Gallienus (r. 253 - 268 AD) Caesar and to look after the western part of the Empire.
Trebonianus Gallus (r. 251 - 253 AD) had his son, Volusian (r. 251 - 253 AD) made co-Emperor, but without any specific territory to govern, more as a "guaranteed successor".
However they were both overthrown and killed by the army of Aemilian.
Decius (r. 249 - 251 AD) made his son, Herennius Etruscus, co-Emperor.
Philip I (r. 244 - 249 AD) made his son, Philip II, co-Emperor (r. 247 - 249 AD).
Pupienus (r. 238 AD) was co-Emperor with Balbinus (r. 238 AD) for only three months, with Gordian III as Caesar.
Maximinus I (r. 235 - 238 AD) made his son, Maximus (r. 236 - 238 AD) Caesar.
Examples like these go back to the time of Octavian "Augustus".
The difference with Diocletian was that he tried to create a co-Emperor who was not a relative, but from merit. And the same for the respective Caesars, to be non related and chosen by ability.
However, almost all of them came from Illyria.
So regionalism replaced nepotism.
For a short time, until the accession of Constantine I and Maxentius in 306 AD.
So whilst Diocletian set out to create a new order, based on an old format, his new order did not last long either.
Yet in laymans history, creating a new, long lasting, order is exactly what Diocletian is thought of.