In my headcanon, the series would have ended with:
Guts, Casca, and their child fight Griffith/Femto. Guts puts up a decent fight, but isnโt strong enough. Griffith tries to attack Casca, but the childโs love from her is so strong that it prevents Griffith from harming Casca. In his moment of hesitation, Casca stabs Griffith through the heart (remorsefully because of Griffithโs connection to her child), and Guts follows up by cutting him in half. In his dying moments, he laments how, despite doing what he always dreamed of and becoming king, the only time he truly felt happy was when he was the Moonlight Child reuniting with Guts and Casca.
The mages somehow manage to summon the 4 Elemental Kings, and they, along with Skull Knight and Danan, fighting other 4 God Hand. SK is the one to finish off Void.
The rest of Gutsโ party, along with Rickert, the Bakiraka, the Elfhelm mages, and Zodd (who joins Gutsโ side) take on the Apostles and the rest of Griffithโs forces.
No idea when the Behelit would have come into play.