system
Given the following post, respond as if you're commenting on Reddit:
human
[General] An extensive research has found that every fictional world eventually manifests itself into becoming a reality in the general multiverse, together with its creatures, technologies and physical laws. Which worlds have now become the biggest threats to the fabric of reality?
gpt
World's that permit movement around the multiverse, as they can spread theur catastrophes. 

Marvel is probably the biggest risk.  It has scenarios that destroy parts of (And the whole of) the multiverse,p and multiverse travel is very common.  

Magic the gathering could be risky too as 'planes' can be collapsed for energy. 

Oddly enough, utter grimdark settings like 40k have never shown much interest in spreading to other realities, but that might change with the option being there.