The key elements include a reimagining of the original storylines. For instance, in a cyberpunk and LOTR crossover, how does the One Ring fit into a world of cybernetic enhancements? Another element is the cultural clashes. When two different universes collide, there will be different cultures, values, and norms. How the fanfiction handles these clashes can make it really engaging. Also, look for how the technology of the cyberpunk world affects the magic or special abilities of the other world.
In cyberpunk crossover fanfiction, you should look for how the different aesthetics are combined. If it's a crossover with a medieval - based world, how do the cybernetic implants and knights co - exist? Also, the power dynamics are important. How do the powers or abilities from one world fare against the technology of the cyberpunk world? And, of course, a good story always has a conflict that is driven by the combination of the two universes.