I bet the entire conviction arc will be crammed into the next movie. Only 3 more movies have been announced, correct? So...that's like one arc per film.
The movie might begin with a 5 minute dream/flashback of the beginning of volume 14, ending with Guts waking up to the Bandits about to kill Jill. Her and Rosine's story will be hardly touched upon (and who cares about Peekaf?), and the fight between Guts and Rosine will be there solely to show some violence early on.
After that boring moth girl fight is over with, we'll skip to Guts seeing Casca burned at the stake in another shaky cam dream brought on by the demon child (rendered in the highest quality 3D animation). Guts will run around a bit with Puck (who has been in the background up to this point), meet Isidro , Serpico and Farnese. Farnese will be infatuated for no reason, because her past encounter with Guts will have been omitted. Luca will be as important to the plot as Jill was.
Suddenly Casca getting burnt by Mozgus! Mozgus will scream a lot. Here comes Skull Knight! A bunch of CG fighting! Guts hugs Casca. The rebirth ceremony will happen for some reason, but hey Griffith is back! More fighting...etc. The movie will end with Farnese saying she will follow Guts as she watches him riding into the sunset with Casca.
That's my 'I have no faith in these movies' prediction.
One thing I would enjoy about a Lost Children movie would be the demi-fans of the other movies wondering why their awesome medieval battle series is suddenly Guts Vs Mothgirl.
Now that I think of it I wonder what fans of the manga at the time thought of the direction everything took after the Eclipse. And for that matter what fans who started with the Dark Horse trx thought when they got up to vol 14 or so.
I wonder whether 4°C will begin to work in the demon baby? I mean if they continue to adapt the series, they can't leave him out forever...
Does anyone know whether/how problematic Rochine/Rosine/etc's nekkidness is as far as censorship? Obviously some naked girls have been in the movies already but I don't know if her situation is different or not.
Lost Children is one of my favorite arcs, though, I can't deny I'd love to see any of it animated.
Given that the manga began with The Black Swordsman Arc I think readers were given enough heads up of what to expect later. Fans of the movies? Well, that might be a problem for them.

I also hope the Demon Child is at least acknowledged, otherwise there are going to be a lot of plot holes.
As for Rosine's nakedness, I don't see it as much of a problem since it is not meant to be titillating. However I am not familiar with Japan's obscenity laws when it comes to underage nudity.