As I said in the summary, I started writing this some time ago (and posted it here, but I can't find the old topic). I was told it was too similar to SCP-093 (which I admit, was my inspiration for it). I honestly feel like the differences I have are enough, but maybe I need to make them more obvious earlier in the entry? Essentially, the nature of the SCP and its differences from 093 become more apparent as you continue to read the expedition logs. If I seem a little bit focused on differentiating it from 093, that's because the only real criticism I got was that it was too similar to it.
The main differences that I'm aiming for are that there isn't actually anything living in the universe that the door leads to, and it's not an alternate future of earth or anything like that.
The universe itself is actually an intelligent predatory entity that manifests 'shadows' like in expedition E2 in order to manipulate the people inside. The entity feeds off of negative emotions, so the subjects being more and more disturbed people makes it stronger and more able to manipulate the people in it. This is why it's not able to directly control one until the sixth expedition, as it had absorbed a lot of energy at that point. The subjects of the expeditions are going to experience more and more disturbing phenomenon, thus creating more negative energy for it to feed off of.
The entity is able to basically read the minds of anyone in it (hence the subject's feeling of being watched in E1). The purpose of the mind-reading is twofold: First, it's able to get information about the subject's home dimension - thus, the buildings and environment seem a little bit more similar to earth in each new expedition (E1 there are buildings but no doors or windows, E2 there are doors but still no windows, etc.); and second, it's able to get information about their past and their fears, which lets it make the trauma more personal, and thus more potent.
In expedition E6, the dimension possesses the subject and manifests weapons that he uses to terminate one or two members of the overseeing MTF and then goes back through the door - this is the dimension making an attempt to lure more people through the portal.
Any critique is welcome!