While most of thile Classic Monsters land seems to be outdoors, there's a large section in the middle which I believe could be covered. This section is labeled B1505 on the site plans:
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From the concept art, it looks like this area is a mix of smaller buildings and possible wooded area:
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The possible wooded area could be covered to simulate a dark creepy forest, similar to the way Knockturn Alley simulates night.
This is a bit of a reach but, it's possible this section is the rumored "walkthrough" area, as opposed to the windmill being both a meet and greet
and walkthrough. There's certainly much more space in this area as opposed to the possible windmill.
Further reach on speculation, but the walkthrough could be a sort of haunted house/scarezone.