I don't think we'll be able to trade the Cosplay Pikachu. Why do I say this? Remember her?:
For those who don't remember, this is the
Spiky-Eared Pichu. Before the release of
HeartGold and
SoulSilver, there was an event where you got the "Pikachu-colored Pichu" (also known as a shiny Pichu) on your copy of
Diamond,
Pearl, or
Platinum. You would trade it over to
HeartGold or
SoulSilver and go to the Ilex Shrine with it (and an empty slot in your team). The Spiky-Eared Pichu will appear, befriend your shiny Pichu, and she will join your party.
This Pichu can't evolve (due to "time-travel", I kid you not) nor can it be traded to other Generation IV games (the data for it doesn't exist in those games). You also can't send it forward to Generation V on (why the hell did they even make this?). The only stats that are different from normal Pichu are the Pokéathlon stats. I wouldn't be surprised if the Cosplay Pikachu has similar properties (can't be traded* nor evolved plus some extra benefit to help it perform better in contests).
Game Freak, just don't let this be another remake-only Pokémon.
*though the data for it might be patched in (and let's not start another conversation about this. I'm just saving everyone else time by mentioning it myself).