New Planeswalker cards tend to get two core set printings, so I'd say Ajani, Caller of the Pride and Liliana of the Dark Realms will be reprinted in M14, while the green, blue and red spots will have new cards and/or reprints from some block. Apparently, the red will be a new Chandra; there's still a chance Jace and Garruk will be replaced for this set.
Jace being replaced? Maybe if enough people request it. Otherwise, not likely. Generally, Planeswalkers originating from blocks like Gideon and Sorin don't get new cards on their first core set printing, they use prexisting cards, likely the first or most recent depiction. Likewise, if Nissa is printed in M14 or another core set before she gets a new card in block, it will likely mean elf tribal. Granted, Wizards makes the rules so I could be wrong, but it seems to me those are the rules they want to follow.
Since Elspeth has already had two PW cards and the latter uses her proper name, is less in-demand for Modern (Elspeth, Knight-Errant is likely to appear in Modern Masters), and synergizes well with most of the white guild mechanics, she'd make a perfect choice for the M14 white walker. Ajani and Gideon have already had their turns, so why not Elspeth too? But Ajani, Caller of the Pride 's only had one shot in a core set so far so chances are we'll have to wait until M15 to see someone new. Likewise for Liliana of the Dark Realms and the spot of black walker.
Koth would be a prime red walker for M14 since he focuses on Mountains and RTR just so happens to have four certain Mountain cards that would allow Koth to be played effectively in nonmonored decks.