I wonder if the OUAT showrunners themselves are fans of Elsa and want her there, or if this is just a mandate to have her on the show from the higher up Disney executives? When the new season starts, Frozen will have been out for almost a year so this doesn't seem like much of a cross promotion, but the movie is not old enough to be grouped with the more classic Disney films. Not new enough, but also not old enough to be a classic. Frozen finds itself in a funny situation as a film.
I heard the OUAT writers really wasted Rapunzel, they didn't give her much to do, so fans shouldn't get their hopes too high for Elsa.