I think people need to give Miura a little more credit.
It's almost guaranteed he's not going to make Griffith a cackling bad guy who wants to destroy the world (at least not intentionally).
Not only would that be boring, but it wouldn't make sense with how Griffith was built up before he became Femto.
His "dream" was his character defining trait (to unify the country/world and become King iirc).
If that's no longer his dream he's no longer "Griffith".
And if he IS following his dream then there's no reason for him to set it up just to destroy it later.
People keep saying "he/they set it all up so it's their fault anyway", but the Kushan still would have attack Midland and the Demon King would have still been worse than their current situation.
Even if Griffith went into explicit detail about everything he did up to this point and the whole country found out about it, they would probably still follow under him (partially because they have no choice).
People are WAY too hung up on the idea that Griffith did horrible things to get to this point therefore everything he did/does after is obviously bad as well.
I'm not saying that he has everyones best intentions in mind (everything he does is most likely for himself as he himself constantly states), but blindly believing in him/blindly hating him are both premature.
- That rant aside, I like them hinting at the fact that Griffith and the "Moonlight Child" might be one in the same, whether it's split personalities/ souls inhabiting the same body, or it's all one or the other (fully Griffith or fully the child) acting as the other.
Though Griffith turning into Femto at that one point kinda discredits that last theory.