That being said, the Skull Knight does distinctly tell Guts in volume 18 that what's about to happen is a "once-in-a-thousand-years" event. So there is definitely a parallel between the two, and the burning brand on the land is meant to evoke that.
That's one of the big questions: if it was an incarnation, then who was incarnated and what purpose it did serve? And if it wasn't an incarnation, then what was it? That's where the thought that maybe all of them were reborn at once comes from, because it would feel like something of an appropriately massive scale.
I see, taking into account what the Skull Knight says certainly enforces that logic.
In that case, if we go with the idea that the current God Hand came to be in a sort of reformative process, and Void's goal is to reenact what was supposed to happen a thousand years ago under more limiting circumstances, but make it succeed this time, wouldn't that mean the incarnation was part of it back then too?
Here's a thought, what if an incarnation was desirable in order to have a replacement for Gaiseric, essentially to infiltrate a member of the proto-God Hand among men. Void would have been the perfect candidate, well-know and respected by the people, after being locked away and tortured by Gaiseric, the unjust ruler, he makes a divine comeback, assuming his rightful spot once again; not too unlike what went down with Griffith in fact. This farce might even explain how Charlotte's tale from volume 10 and Mozgus' anecdote from volume 18 about divine punishment got passed down.
There are holes in this train of thought, naturally. Why would Void sacrifice the very people he would want to infiltrate. Did he intend to sacrifice all of them, or only the ones who followed Gaiseric. Was the whole capital destroyed as a result of the sacrifice, or in the aftermath of the battle between the two forces.
Since Void is where he is, if something like that were to have happened it clearly failed, but how did it fail, was it interrupted in a similar manner the Skull Knight tried to interrupt Femto's incarnation, or did something else go wrong?
The beauty of fiction is that you can take any element from the real world (a name, a design...) and assign to it a new signification. [...] So we shouldn't limit the possibilities we envision for the story based on that sort of thing.
I feel the same way, believe me. I know there are people who can't enjoy works of fantasy without anchoring everything to the real world and being annoying about it, and I'm not trying to be one of them.
In a more practical sense though, Berserk is still a heavily visual work and not everything it depicts is bound to have a deeper significance, even under the premise of fictional interpretation. I think it's worthwhile to make that distinction when possible and not latch onto false trails. Ultimately, I don't want to limit the extent of anyone's imagination, just to provide some background since this stuff interests me personally, and I absolutely love the fact that Miura incorporates it into his work.
My justification for thinking this is the nature of those who have been sacrificed and the fact that they are not necessarily feasted on, rather it's as if their life force and/or soul is being stripped from their bodies. The remains at the bottom of The Tower of Rebirth were essentially intact and Gaiseric's presumed lover dies in his arms with no visible mortal wounds. There appears to be some physical damage to her in the form of bruising or blood splatter on her face and neck, but it is no where near the carnage that usually comes as a result of being branded.
This is an interesting idea. It crossed my mind as well that due to the original ceremony having a different nature, being "closer to the source" in a way, those who got branded experienced a different kind of death. After the introduction of apostles as we know them, the feasting might have evolved as a method of compensating for that.
This of course leads to the real question of who are the other 4 individuals standing alongside Void? Outside of prior God Hand members or those loyal to the human who became Void are they perhaps malevolent deities or the antithesis of the Four Kings of the World?
The light in which those four got introduced heavily suggests they were previous God Hand members, but in any case, I don't see how your other ideas are dependent on them being something else.