As Mark Rosewater and other R&D members reiterate time and time again, although planeswalkers are a major feature of every current set, their design space is very precious. After all, it's hard to innovate on a card type with such specific constraints as planeswalkers have. But one unexplored area that many a custom card designer has mooted is "planeswalker teamups" - Usually expensive and splashy two-subtype cards representing two characters on the same card. Often these custom designs can activate two abilities per turn, or require a planeswalker sacrifice to cast, or some other flavorful "this is a duo" mechanic.
And custom designers aren't the only people thinking about it - the real deal R&D has considered it too, according to Rosewater. When chantryg asked him on his Tumblr blog, "How would you feel about a card that represented two Planeswalkers teaming up? Planeswalker - Jace Chandra, perhaps?" he responded:
It’s an interesting idea. We’ve talked about it. Maybe one day.
Of course, the big issue at hand is how to get a double planeswalker to feel like it's really two people on your side while making it both fun to play and balanced. The two-activations-a-turn thing seems like the best way to do this, but would take a lot of testing to get right; with sufficiently strong abilities, such a card could be format-breaking.