Something there is that doesn't love a Wall. His name is Mark Rosewater, and he's against that creature type on principal. Still, all his years at R&D have certainly taught him that creatures with defender are fine and necessary for the game, and Walls do continue to be printed since R&D is a team effort and many of the people on it do like the Wall type, even if Mark feels that an architectural feature shouldn't be a creature. Rosewater recently got asked a question by a Wall fan on his Tumblr blog, and the response was, well, pretty massive.
When zeroephyrt asked, "Is a legendary wall on the 'list'?" Rosewater made it an open question, so to speak. And, for his troubles, got more than 300 notes on the post at press time. For a creature type that's been around since Alpha, it's pretty odd that there hasn't yet been a legendary Wall representing a named city's boundaries or something to that effect, which many of the comments pointed out.
A lot of those comments said something about the resonance of the Great Wall of China and Hadrian's Wall as historical tropes that could easily be fictionalized as a Magic card, and there have been enough custom cards for a "Great Wall of Tarkir" on MTGSalvation to prove the demand for it in the next block specifically.
My take on a legendary Wall on my own blog: "Could be a good defensive white mythic legend (to replace the stereotypical Angel). I like the idea of one that grows and can block more with each creature blocked. Possibly a fortress with multiple types of siege/army/fortcrusher weapons (offensive activated and triggered abilities) as well."