BOOM! GOT IT!
rem and ram! what the hell happened to two orphaned girls ended up under roswaal? fill in the story! so, their town's burned down by witches cult, everyone's killed, etc. the two go through a grueling adventure making it back to civilization, a chase by witch cultists to a hiding spot, having to survive in the wild for a week, then trekking to a city and having to fight monsters, and people(?), and maybe more witch cultists. what i'm saying is that the first leg of their journey will just be them getting to a safe area...alternatively you could have them picked up and go in a totally different direction with some political thriller and drama and them slowly training and developing, ultimately meeting roswaal, who takes an interest in them, and eventually takes them in after tragedy befalls them or when they prove themselves.
i know in my "thoughts" chapter i said some stuff about how it's hard to write without a lot of source material, but this is a bit of an exception. with a clear beginning and conclusion, you just need to fill it in, add some lore of your own, build a world!
anyway, they get to a city, at this point i think the mood should shift, reflecting re:zero's own distinct changes, much like charlotte, but less compressed. the two should spend time learning to be maids, gaining skills, making a few close friends, and seeing possible enemies. it's up to you how this plays out, as always, but i'd like to see one or two close friends who die or have to leave at some point. you could make the story light-hearted, which would be great, but it might be better to craft a darker tale. so, they get some training and then have some power struggles, back alley fights, maybe gang wars? the point is, they get caught up in some stuff they didn't see coming, through whoever trains them, where they live, who they're friends with, doesn't matter. you could even kill off the close friend(s) here, but i'm not sure the bond would be strong enough to break meaningly.
so far: return to safety, training, war, and that brings us to the final arc, or you can add more. we don't even really know how long this time period is, do we? you'll probably have 1 or 2 major timeskips. so the final mini-arc is them proving themselves to roswaal in some way, and becoming his maids, or servants, or assassins? whatever they are...
so yeah that's the idea, fill in the gap!
p.s. i feel like i updated almost too soon.