When will we find out that Griffith's eating the souls that he summons for his little "show's".?
If things never end up showing Griffith as a villain anymore, it'll fit this manga's theme of Guts never being able to be a winner, he won't stop until Griffith's dead, so we know eventually, it'll be Guts vs Falconia, destroying it outright for vengeance and hatred.
If it does turn out that Griffith has a grander scheme in play, I find I'll enjoy it just as much as if he didn't, it'll play out the same way anyways, with Guts looking like a bad guy to the world, and Griffith looking like a late great hero, only this time, we'll know that Guts had saved the world (probably at the cost of his own life.)
On a side note, unless Guts kills ALL the demons under Griffith's rule before Griffith, he'll unleash a new hell on earth, but at the very least, assuming Griffith's death stops the two world's from fusing, the new hell will only be the apostles, and not the entire supernatural realm, so it's a lesser of two evils.