no one knows what the 20 sign is about except the dev, AFAIK. The current assumption, again, AFAIK, is that its going to lead to something in the final game, as there are similar rooms with signs in Battle Girl and, i think, Doll.
As for the rest. Its smaller in scope than Battle Girl. I agree, some parts feel unfinished or rushed, but i'm okay with it. Fuyou has turned out 4 games in 3 years. That's damn impressive by any stretch. Yes, EP Girl is shorter than the other three, and Hero Youth is shorter than Battle Girl. But you can see a clear progression in game development and art from Doll to Hero Youth. Add in the fact that he's weaving a narrative across the games with no dialogue, a few pictures and a few cut scenes, it boggles the mind.
These games intrigue me. Its why i'm actually posting instead of just lurking.
So, i agree, it feels like he had places he thought "OOh, this would be great here" but, IIRC, he posted that the project grew out of hand, which happens to creators quite often. I mean, its a weird comparison, but look at A Song of Ice and Fire by G R R M. It was originally supposed to be three books. Then it became 5, and then 7, and now it doesn't seem like it will ever be finished because he just kept coming up with more things to add. So maybe we should be happy Fuyou gave us a finished game that may not have all the things we would like, but is finished, rather than a slow drip that gets slower after each update.