Some pieces of fiction have a "reveal" of sorts. For example, Darth Vader is Luke's father, Snape was working against Voldemort, witches are fallen puella magica, Soylent green is made of people, etc.
But how should fanfiction treat this? Especially when its a crossover where the characters crossing over have no idea what the reveal is going to be? Should the author attempt to build tension? The problem here is that, likely, the person reading the fanfic ALREADY knows the reveal, and as such, most of the tension (which is for the sake of the audience) is lost. If tension is to be built, then how? Foreshadowing, one of the best methods of building tension prior to a reveal, is completely pointless in fanfiction.
Note, this is not the same as "prequels". Prequels can get away with building tension by introducing a while new plotline. For example, star wars ep 1 to 3 had the clone wars, padme, jar jar binks, etc thrown into it, not just the fall of Vader.
To put it another way, imagine if you've been hired to write the script for a soylent green remake, how would you go about it? Keep in mind that nearly everyone on the planet already knows that Soylent green is made of people, but your job is to make the new movie as surprising as the old one (barring cheap tricks, like over the top special effects or turning it into a slasher fic).