I guess a good "use" for Guts, according to their intentions and what Daiba said, could be treating him as a magnet / attractor to trigger and speed-up the happening of the final confrontation (?)
Not that this makes sense for what is established on Miura's Berserk, but I believe it don't need to make sense for the Continuation:
Use Guts as a magnet to me seems to be what Daiba is heading for, but why? Well, under Ganishka command he created the man-made behelit with many captured apostles, obviously it was the later Emperor who captured them, neither Daiba or any other Kushan was capable of this, maybe a horde of Daka could but first they need the pseudo behelit to have Daka.
Who else we know that can subdue apostles? Not only that, but also got a match against Ganishka true form and (with a fluffy help) held a finishing attack to it and make him retreat(I know I know that was situational, but still).
But why? We know that it could only be achieved because it was powered by Ganishka's fog, but does the continuation know that? Even with Miura I always had a felling that the man-made behelit
concept would be used again, first because it was the key to the merge of worlds, and if it's needed to revert the state of the world some like reverse engineering could have been done with this concept, and second because it's use was capable of creating, possibly, the most powerful being below the God Hand. I still don't see any member of Guts co. agreeing with such a thing as becoming what they fight against, but this was the only time we saw anything close to manipulating a behelit usage before.
Sure I have many holes in this theory, we don't know what the magicians of Skellig were capable of and how their magic works, but we had tips about Volvaba that can manipulate departed souls(how that relates to corrupted souls of apostles?) and Molda might also know something about this field.
So while I know that the man-made behelit can't be replicated, it's concept can be used to create another thing and be used together with other different field of magic, and I believe such thing would have been used by Miura.
But now the Continuation team can only throw some shit at the ventilator without proper explanation and, as they keep doing over and over, contradicts Miura stablished facts. And I believe it's what will happen the next few episodes in Daiba's plan.
perhaps to protect Kushan from all the magical creatures
Which creatures? lol Fantasia did not even reached Kushan territory yet by what we know
Then Guts is reborn as some kind of Demon-human hybrid
I believe that's too far even for Continuation standards. Guts can have power ups, sure, but not from a source like the God of the Abyss or Apostles, that are what he most hates and antagonizes. That's a principle since ground zero, Guts can become a monster of his own, but he denies the essence of God Hand and apostles-like.
or his sword just gets more powerful after an encounter with pure evil and becomes capable of damaging god hand (again, all bs stuff)
I can see they going in that path though, if SK infused his sword with behelits they can see that as something replicable by a man-made behelit. But indeed, it's all bullshit.