Alright, so I've got three ideas for people's consideration.
This first one I think is really interesting, but it might end up being too derivative of its inspiration. It's inspired by a book called The Midwich Cuckoos, wherein an area is affected by possibly an alien life form (possibly something different in this SCP), and all females are left pregnant with children, who end up being a different species which possesses mind control. In the case of this SCP, I might make their mind control either milder or have a countermeasure in order for the SCP to be feasibly contained. The details are all up for change in case the idea is too derivative or bordering on plagiarism, but the main idea I want to preserve if the idea of there being a human brood-parasite.
The second idea is a bit more original. The idea is that some agency (might leave the specific name of it redacted) sent up a secret space station to study alien life, and upon finding a rock floating around earth with some microbial life, they bring it into the station to study it, where after it grows and comes to resemble a fungus, it breaches its containment on the station, coats the inside of the station with a completely similar fungus, and infects everyone working there, unifying them in a hive-mind of sorts. These events would likely be found out either via a log on the station's black box or through an interview with one of the infected, who are either contained together on the station with the rest of the contagion or contained separately on earth.
The third and final idea is decidedly simple and could probably benefit from a bit of development. It's essentially simply an Escherian Stairwell (also known as penrose steps) in an abandoned building in an abandoned town somewhere in the middle of northern Siberia. Every time an individual goes up the stairs, they're seen coming up from the same stairwell, and when going down, they are seen coming down from above the stairwell.
Thanks for reading through my ideas! I look forward to people's input on them.