Subject: You should probably keep the missions self-contained.
Author:
Posted on: 2014-02-04 05:26:00 UTC
Introducing characters from other places in the MLP multiverse would more likely than not either end up confusing people or seeming unduly shoehorned in. Dealing with a single badfic all the way through, only for a character from another more respected fic to come in at the end to solve the problem would require some background explanation in-universe, probably with some sort of previous setup involving that character. Otherwise, it would just come out of nowhere, akin to an Agent pair in Star Wars saying "Hey, super-Sith Sue! You're too powerful for us, but not too powerful for Yoda! Ger her, Jedi Master!"
One of the reasons canon characters don't know about the PPC, from a meta-fictional standpoint, is to prevent the Agents from delegating their tasks to someone else, or from having pre-existing allies that they know beforehand can help stop the badfics, both of which run the risk of unduly increased speshulness. Going out to enlist the help of a well-known fanfic character runs into a similar problem, with the added issue of the reader possibly not having heard of or read said character's fic of origin. Nyx would need to be established primarily through the Agents' back-and-forth, to make sure that everyone is up to speed, and frankly, she sounds very Suvian when going by her description alone, which would be problematic regarding the perception of the readers.