SCP Foundation

Me and my brother first talked about this idea back in 2014, when we visited Fort de Vaux. I'd love to do some sort of skip which is an French fort from World War I with a lot of creepy, anomalous stuff that goes on inside (and the tunnels underneath…), but I can't really think of a way to make it interestingly different than SCP-455. Thoughts?

With what you have, you are only describing the setting. You still need the actual anomalies.

Consider what kind of anomalies would appear in that place, and focus on what makes them stand out. Ghost stories involving WWI and WWII sites are nothing novel inherently, so you have to stand out from even them.

Instead of wondering what anomalies to put into the SCP, an often piece of given advice that I have noticed and believe applies to you is to come up with a story that you want to tell first. It is better to have a story that your anomaly fits into than it is too have a anomaly with no story. The best example of this would be SCP-093. The object itself is just a portal to another world and if you remove all but the basic premise it is not all that good. But because the portal just serves as a set up for the actually meat of the article, that is this alternative world for the foundation to explore and the story of what happened to this other world.

SCP-186 is a very good example of a WWI era SCP