Despite many claims that claims G5 is an alt university due to lack of continuity (never mind G5 taking place god knows how many years after), it has shown the occasional shout-out to G5’s lead writer and story editor Gillian Berrow’s work on G4. Among those are giving a name to Celestia and Lina’s hometown, glowpaz (which also appeared in FIM later on), and more recently: a hidden toon in the Crystal Lighthouse much like Starlight Glimmer and the Secret Suite.
My thought? It would be Opaline returning, finding the Shadow Helm, and becoming Nightmare