Hmm, I don't think it would be a new mechanic as BIG as mega evolution was, perhaps a more "quirky" mechanic, but items related to the legendaries seems likely to me, and the fact that they are opposite colours to the logo might be similar to what was going in with the legendaries switched in Black and White.
Maybe a special kind of transformation stones, similar to Mega Stones, a la Orbs for Primal Reversion?
Maybe Lunar Evolution and Solar Evolution, which the two main legendaries go through?
Maybe the one in Sun goes through Lunar, and the one in Moon goes through Solar, or something like that?
Maybe a special kind of transformation stones, similar to Mega Stones, a la Orbs for Primal Reversion?
Maybe Lunar Evolution and Solar Evolution, which the two main legendaries go through?
Maybe the one in Sun goes through Lunar, and the one in Moon goes through Solar, or something like that?