Mf formed a tetrarchy.

He saw that the empire was simply too big to for one emperor and nominated a co-emperor and both together nominated juniors called caesars.
Hence the name tetrarchy.
Both caesars were to become augusti themselves and nominate new juniors to follow them.
The choice was to be made by ability and merrit.

After diocletian retired and forced his co emperor to retire with him, so the tetrarchy could work, family loyalties caused the death of the system. It was never tried again.

Diocletians failure was not to see, that the aristocrats were unwilling to ignore family ties.

Btw.
Sulla wasn’t an emperor but considering in his time he had all the strings of the republic in his hands, I’d like to add him to the list.
He retired to his villa in naples in lived his days out, if we believe our sources, in a litany of sexual perversions.