When planeswalkers get multiple card versions, there is a design challenge in making sure the characters have a consistent identity - most planeswalkers specialize in a specific type of magic, after all - while having the cards play appreciably differently. It's important to make sure, for example, that Garruk is always an animal summoner, Gideon is a law mage... and Liliana is a necromancer.
That last character hit a bit of a wrinkle with her latest card version, Liliana of the Dark Realms - this Liliana card still very much feels like she's a corrupting black mage but her card has nothing to do with dead creatures or graveyards at all, only with Swamps.
Mark Rosewater feels strongly about this, and recently addressed the issue on his Tumblr blog, Blogatog. When mkfffe asked, "I'm a big fan of Liliana. I love necromancers. Which part of Liliana of the Dark Realms was her necromancy or was this something different in her personality we haven't seen before?" Rosewater answered:
Liliana if the Dark Realms is considered a mistake because it doesn’t match her powers as a character. It was that card that got us to map out the flavor and connected mechanics of each planeswalker to prevent a repeat of that mistake.
If R&D in general doesn't like how the Dark Realms card fits Liliana's character - if anyone should have had abilities like those, it'd be Koth, with the references changed to Mountains and the second ability to +X/+0 or X damage, obviously - then she's close to a shoo-out for Magic 2015. So assuming Liliana's in the upcoming core set - and promotional material suggest she very much is, as opposed to, say, Sorin - then it'll be with a different card.
Liliana of the Veil is considerably too strong and complex (see her "divvy" ultimate) to print at a core level, so it's either the uninspired choice - Liliana Vess again for the first time in four years - or a completely new planeswalker card for the dark necromantic seductress.