Yeah, I see what you mean, the kind of thing I'm thinking about is collapsed sections and tunnels, for one, it would make sense for there to be some (moria being so ancient) and also, it just makes the hair stand up on the back of my neck to think of a huge open space behind that rubble pile in the doorway, or through those cracks in the wall.
We wouldn't need to make the space behind a so called rubble pile inaccessible, but have that particular entrance blocked up. With my original idea however, this kind of thing would be at the very bottom of moria, in the deepest recesses of the mine, probably taking at least an hour of walking to get down to these areas it would be a creepy feeling that moria goes even deeper. And just to be clear, we wouldn't block off completely any halls that are going to be made in detail, i.e, we wouldn't subtract anything from the colossal size we already have, we would use this stuff to add detail and give moria realism. I'd actually think it unrealistic without this kind of thing, the first reason being that of course moria would be damaged heavily in some areas, and the second is that in the books Tolkien seems to describe moria as something that would take more than a life time to explore, of course in mc we can't make that happen, but we can make it feel that way.
You get what I'm saying? To put it in a basic form, say if moria was a straight hallway, that was it, instead of having a simple dead end, we could make it seem like the hall goes for longer, (we wouldn't be shortening the hall thats already there) but the rest (that doesn't actually exist) had collapsed. I totally agree that it is kind of annoying to be exploring a build like this only to find parts you cant access, but I think it's equally irritating to run into a flat wall at the end of all the work, but showing that you cant continue because the passage had collapsed gives a more interesting feel.