My take:
The overarching theme is that fate is unavoidable (except for Guts). Griffith was fated to become Femto. If he weren’t imprisoned, he’d have probably married Charlotte for power, briefly become a prince, then defended against the Kushan invasion, at which point he’d probably sacrifice his army in a moment of desperation after seeing the power of the apostles tear through his kingdom.
If they didn’t invade yet, it’d happen due to political betrayals from other nobles (I.e. assassination attempts) who were against the wedding and didn’t think a commoner should be a prince. If that didn’t happen, his obsession over Guts would make him go crazy and reckless and his guards would need to depose him.
He was destined to become a God Hand, so fate would probably line things up with the eclipse. Apostles already existed in the world and once Griffith saw there was a higher level of power, he’d have wanted it. He’d probably even sacrifice Charlotte for it. Imagine Guts and Casca hearing of an invasion or coup (etc) and returning to visit the band only to see the kingdom transformed into a literal hellscape. Maybe they’re brutalised by a monster and only learn afterwards it was actually Griffith.
Imo it’d have been much cooler to have seen the royal politics or a foreign invasion play out over a separate arc, rather than just him being a window-climbing rapist and getting tortured because his sword was broken, lmao. Seeing him actually briefly attain his dream and watch it die a year later would make his fall more dramatic and meaningful.