Yeah ia a time skip seems really likely. If I had to guess I’d say maybe 10-15 years?
If it’s like, a century then Guts would come back to Griffith running an empire as an immortal god-king, all the characters we know like Rickert and Charlotte and Silat etc would probably be dead, and I think it would change the tone of the story too much. Like yeah it’s an epic story, but at its core it’s about relatable characters, and if you start kiling those characters off for no reason other than to add to the epic-ness then it throws the balance off.
I could maybe see 50ish years, making Rickert an old wise mentor like Godo lol, and maybe aging Sonia so that she reminds Schierke of Flora? Could be interesting. But overall I’d probably prefer less time, that still seems like too much.
10-15 years gives Griffith some time to start acquiring his Gaiseric-esque empire, it’s enough time for the new fantasy reality of the world to change how things work and function, it’s enough time to Rickert to grow up and do whatever he’s doing up in the mountains with Silat and co (resistance movement sounds good tbh), enough time to make everyone sad (like say if Farnese’s parents died in the interim or something, or Isidro had a family waiting but not anymore), but it’s not so much time that the world Guts returns to is unrecognizable and without familiar touchstones.
Also just to be self indulgent for a sec, several decades is too long for NeoGriff to be chill and serene and doing his thing without Guts despite his unfrozen heart imo. It makes the whole beating heart that he blames on the fetus thing seem irrelevant if he can easily function for that long regardless. But a decade or so sounds like a perfect range of time for NGriff to start out serene, throwing himself into his empire etc and pretending he doesn’t care at all about Guts, only for the cracks to begin showing just as Guts is set to return.
Of course if he is able to put Guts out of his mind for 50 years until he’s standing in front of him, unaged, and then he has an emotional response, I’d still be okay with that. It’s just a little less fun to me.