You lack vision.
After all, theoretically, the galaxy and more specifically humanities existence is at threat.
Practical solution is to form a regional hegemony to act as a means of reducing Xenos threats and preventing outright domination by rival human empires of a more self-destructive nature.
Looks at Angron and Curze...
(Typos.)
Nah, it's more that Guilliman, as a character, doesn't appeal to me the way other Primarchs (like Curze) do. Putting aside the wole "vanilla marine" thing, Guilliman as a character is defined as a steward/manager, or, if you want to be cheeky, the ultimate second in command. He's always been most at ease making systems and managing the state of the realm, rather than playing visionary. The systems he set up on Macragge were the ideals of his father, he rarely innovated or pushed boldly forward, preferring instead to fine tune and improve things that others made. The Moritat came from Corvus Corax, the Fulmentarus Terminator Squads were originally Perturabo's idea. What's more, Guilliman has a tendency to be methodical and conservative, most notably displayed in the Imperium Secondus, where instead of going to look for the Emperor, he assumed the worst and started fortifying and preserving what he had in reach. He refers to his laws and systems first and foremost, even at the cost of ingenuity and initiative. It's a flaw that is passed down to his Ultramarines, as shown by their steady entrenchment of the Codex Astartes (which Guilliman specifically stated was only a set of guidelines) into dogma over time. Guilliman knows his flaws, of course, but that doesn't make them any less prominent in practice. His self awareness and objectivity is one of his strengths, hence why he made Sanguinius Regent of the Imperium Secondus, and when he was awakened chose to play Advisor rather than take center stage. As long has he has enough power to do what needs to be done, it's enough.
Ultimately, Guilliman is a orderly, practical person first and foremost, and writing about his airtight policy making and methodical expansion isn't interesting to me. I'd much rather write about Konrad Curze's, and his struggles with his inner monster; his attempts to bring light to Nostramo (metaphorically through stability, re-education and intimidation, literally by turning the moon into an asteroid ring via shaped cyclonic torpedoes); his reaction to things outside his original perspective, and how he reacts and grows in response. To me personally, Curze is a more compelling character who doesn't get enough representation as is. Besides, you're going to be playing Guilliman, so it all checks out anyways?
Also, my inner Rogal Dorn chuckles at the term "you lack vision", because A) a lack of "vision" isn't actually a bad summation of Guilliman's biggest fault, and B) Curze lots of visions. He had "vision" spilling from his ears, to the point that they were a big reason he went insane.