IF Eleanor turns out to be a popular leader/dual leader/pseudo-leader, then nothing will stop the Firaxiods from giving us more dual or triple-civ leaders wherever they can. If, on the other hand, she is met with resounding lack of interest, we'll hear no more about dual leaders of any kind.
Either way, I think it misses the point that 'dual leadership' results from either conquest (Kublai, Alexander) or Royal Breeding which is more enthusiastic than selective (Eleanor, the Hapsburgs, et al) and so neither, in game terms, currently exists at all. Conquer another Civ or City State, and it all becomes One Civ with One Leader - no duality possible. And having Immortal Leaders in the game means there is no Royal Breeding of any kind, so that's out, too.
Frankly, even Eleanor is a 'gimmick' Leader: it all depends on what she brings to either England or France to make her worth playing: her origins are immaterial to the game.