Here's an idea of a memetic SCP (no, no, come back here; don't automatically flee from it just yet!) that I'm unsure if was already done in this site or not. It's a "memetic worm". Or, to avoid confusion with the zoological sense of the word, a "memetic botnet".
SCP-WHATEVER-1 is an abstract symbol that started out as, say, a grafitti tag. Anyone who sees the symbol is infected, and feels an unconscious impulse of replicating the design. Imagine a student scribbling on his desk during a boring lecture, or someone sketching on a post-it while on the phone would come up with this symbol even if they don't remember ever seeing it. Then it is left in open view to infect more people.
SCP-WHATEVER-2 is an indeterminate object that is the "key" to the memetic botnet. Someone with the object on their hands can issue a command or a concept to the minds of all the infected, and all these people will accept that command or idea as their own, as if they really meant to do it/actually thought of it by themselves.
Ring any bells?