The twentieth anniversary of Magic will be marked by this year's core set, Magic 2014. We don't know much about its contents yet, but each core set brings us certain things we know for sure. There will be a lot of reprints, and one of the mythic slots in each color will be devoted to a planeswalker.
The lineup of the five planeswalkers, however, is subject to change at R&D's whims... within limits. New planeswalker characters would not be debuted in core sets; but that doesn't preclude new card versions of those characters from being printed. There's a reasonably wide roster for R&D to choose from, so which of them will be in Magic 2014? Here are some educated guesses:
White: Gideon Jura
There are three planeswalker types that have been on monowhite cards so far - Ajani, Elspeth, and Gideon. Any of them could foreseeably appear at core (Ajani's armies-and-lifegain abilities, Elspeth's tokens-and-mass-destruction theme, and Gideon's turn-into-a-creature focus all make sense in a core set), but only Ajani and Gideon have thus far. Personally, I think it's due time for Elspeth to get her day, but the fact of the matter is, Gideon is much more in focus right now. He's one of the heroes of Return to Ravnica block, having put in an appearance fighting for the Boros Legion in his Gideon, Champion of Justice guise. So it's likely that his original version will put in an encore appearance in Magic 2014.
Blue: Jace, Memory Adept
Call it a lack of creativity on R&D's part, but there are only three monoblue planeswalker characters - Jace in all his versions, Tezzeret the Seeker, before he fell into Bolas's clutches, and Tamiyo, the Moon Sage. Tezzeret's artifact-related abilities are simply too niche for a core set planeswalker, and that theme is too integral to his identity to rework. Tamiyo's abilities are much more fitting, but she's a Kamigawan Soratami, and her nonhuman appearance might be too alienating for core. That leaves good old reliable Jace to return for the fifth core set in a row. It'll be time for a change soon, perhaps, but not now.
Black: Liliana of the Dark Realms
Sorin Markov has his charms, and they were enough to score him a core set appearance in Magic 2012. But Liliana is just too perfect of a black planeswalker - she's manipulative, deliciously amoral, seductive, and possessed of vast necromantic powers. Sorin's tortured heroism is nice and all, but Liliana is basically an entire color boiled down into a single character, which is a great way for a core set to get the message about black's color philosophy across. Her version from Magic 2013 has old-school flair and shows how black can spread corruption and use that to its advantage.
Red: A new version of Chandra
This is a known quantity, given the preview art in the Magic 2014 announcement. We don't "officially" know that Ms. Nalaar is getting a new card in this year's core set, but it'd certainly be a letdown if she didn't. Hopefully it'll be a bit stronger than Chandra, the Firebrand; there are plenty of new places that R&D can take her fire magic.
Green: A new version of Nissa
Garruk has had a good run. But he's one of two monogreen planeswalker characters, he's gotten four versions (on three cards; Garruk Relentless has Garruk, the Veil-Cursed on its transformed side), and the unloved Nissa Revane only has one. Her dependency on Elves (and a specific other card, Nissa's Chosen) made her deeply unloved and we haven't heard a word about her since Zendikar. What better time for R&D to redeem her character than the twentieth-anniversary core set? Mark Rosewater even coyly hinted at the possibility on his Tumblr; when user swmystery27 asked him, "If a 'walker's first incarnation is recieved badly (eg Tibalt/Nissa), does that generally make them more or less likely to reappear? One would think less, for obvious reasons, but is there ever a desire among R&D and Creative to bring them back so they don't get a raw deal?" his reply was:
We try to conserve our planeswalkers, so the audience not liking their first card does not mean that we might not try again with a new card.
To that end, it's distinctly possible we'll see a new Nissa in Magic 2014, albeit one slightly divorced from her tribe-boosting roots.