I've always believed that had Dioclentian's succesors had as good of a grip on imperial rule as he, the Roman Tetrachy could've served as a strong administraive base for another round of Roman expansionism.
That being said, the challenge here is to devise a plausible way for this happen with a PoD(s) occurring anytime during Dioclentian's retirement.
Bonus Points if you can postulate what sort of acquisitions might've been in play had this occurred.