If I were Miura, I would just have Guts and Casca happily retire in the Island forever and call it a day. Ten chapters at most and I'd have all these dumb fans off my back and the rest of my life to write whatever I want.
But Miura just gave Griffith teleportation, and where would Griffith teleport to?
It is obvious that Miura plans to have Griffith use his army to invade Elf Island in the future. Seeing all those loli witches burning and raped by demons will reignite the fire in Guts' soul, which will lead to him destroying Griffith's army and Guts leading all fantasy creatures to destroy Falconia.