Ever since Time Spiral block, Magic's Creative department has explored various ways of integrating characters from the Weatherlight Saga and earlier into the world-hopping current era of Magic storyline. Nicol Bolas, for example, exists as a major villain in the background, having survived catastrophe after catastrophe with his planeswalker spark, while Karn - who technically carries a special, double spark (now possibly triple thanks to Venser) - was revealed to have been trapped by Phyrexian forces during Scars of Mirrodin block. Since both of these reappearances were very popular, we should be seeing more of them in the future.
When kerrjf asked Mark Rosewater on his Tumblr blog, "Karn is the only planeswalker from the past to return to a recent set. Will we see more characters from magic's past returning with the planeswalker frame? (P.S. Please please Jaya Ballard, planeswalker.)" Mark responded:
Nicol Bolas also counts. What characters from the past (that haven’t “died on stage”) would you want to see?
The post quickly accrued 85 notes, most of which were suggestions along those lines, and overwhelmingly sentiments swung in favor of two particular (ex-)planeswalkers: Jaya, the oft-quoted fire task mage of the Eternal Ice era on Terisiare, and Teferi, the Zhalfirin court conjurer who was a part of almost every single event of the Weatherlight saga, as a Tolarian Academy student during Urza's youth, as the chief opponent of the villainous Kaervek during Mirage and Visions, and one of Urza's eight planeswalker allies during the Phyrexian invasion.
Other popular answers naturally included Urza himself, Yawgmoth, and Freyalise, all of whom were directly depicted as having died during the storyline, and Michiko Konda, Squee, and Ugin, who conceivably could still be alive (Michiko having partially fused with the Worldsoul of Kamigawa, Squee being completely invincible and immortal, and Ugin supposedly killed by Nicol Bolas, for which we only have Bolas's own untrustworthy word).