A builder of Empires and a civilized organiser at heart. Guilliman is meant to embody some of the better aspects of the Romans: That of ability to adapt by organizing themselves systematically, and of efficient rule resulting in winning the peace.
The breaking up of the Legiones Astartes into Chapters mirrors the slimming down (and inflation) of Roman Legions during Late Antiquity. The good governance of the Realm of Ultramar mirrors the skilled rule of Augustus Caesar after the civil wars.
Having Guilliman as the saviour of the Imperium after the Horus Heresy always made a lot of sense: Much like the Romans (Byzantines), the Imperium is able to carry on against staggering odds at numerous fronts, and occassionally carry the offensive thanks to sheer ability to organize itself efficiently.
A Roboute Guilliman might never have conquered Terra from the Techno-Barbarians. Or he might have succeeded, and carved out a medium-sized empire just like the Realm of Ultramar but in Segmentum Solar. The Emperor's singular vision and drive to conquer the
entire galaxy rapidly is peculiar to the Emperor, for better or worse.
Probably worse. Inter-state competition can do wonders for technological development and drive to outdo the enemy by expansion, as was seen in both Warring States China and early modern Europe. One monolithic Imperium killing its potential future rivals in the cradle also destroyed any alternative paths of human regrowth as galactic power(s), and the cutthroat competition and drive to outdo the others which would likely have ensued.
Unity is not always a strength. Getting saddled with a single state with an inability to invent is a cardinal sin in the long term judgement of interstellar empire: The whole arena hinges on technological and scientific advancement. And so mankind will be fodder for the Great Devourer and the awoken Necrons as it scrambles desperately for scraps of ancient
STC to save it, and prays for divine intervention.