Magic's planeswalkers tend to stick around for a while after they're introduced. With a few notable exceptions they're intended to, and do, get multiple versions. Of course, this means the originals - the Lorwyn Five - have appeared in a string of core sets unbroken save for Magic 2012, where Gideon and Sorin filled in. The original 'walkers fit the bill as characters who are most often monocolor and have personalities that exemplify their colors' philosophies. But storylines change, characters develop, and the Lorwyn Five can't stick around forever. Here are five templates for monocolor planeswalkers who could replace them someday.
White: Both Ajani and Gideon - and indeed Elspeth, the last monowhite planeswalker - are unambiguously heroic. But white lends itself to prejudice, fanaticism, and totalitarianism - all traits that could appear in a villainous planeswalker who believes themself to be doing good. It would be interesting to see a white bad guy in core: A Judge Dredd-style law mage who believes all crimes are equally heinous, or a religious zealot of a phosmancer (or light mage) who wants to cleanse the Multiverse of sinners.
Blue: Jace has his fair share of detractors, because he exemplifies a hated aspect of blue - the idea that blue has mind magic and thus can control anything anyone does is what led to the historical overpowered designs in the color. But blue is more complex, flawed, and interesting than that. A "knowledge for knowledge's sake" character like Tamiyo would be a good core set introduction to what blue believes, but so would a planeswalker representing the sea (Kiora turned monoblue or otherwise), mercurial, inexorable, bounded by unfathomable precepts.
Black: The two planeswalkers who have been monoblack are Liliana, a necromancer; and Sorin, a Vampire. Black does have something to do with death, it's true, but it shouldn't be pigeonholed that way. It's nice that Sorin is a hero; but a more neutral black-aligned character would be even more interesting. Something on the order of a selfish interplanar merchant or banker, for example, or simply a "swamp thing" type of pseudo-monster 'walker that corrupts because it's in its nature to.
Red: As well-loved is Chandra is, she's also kind of lazily, stereotypically red, as well as derivative of Jaya Ballard's character. While the more levelheaded, but still emotional, geomancer Koth would be fun to see at core, so would a groundbreaking and much-requested Goblin planeswalker - a jester or chieftain or what-have-you.
Green: So we have Garruk, animalistic tracker and summoner, who's been pulled away from monogreen to black-green for the foreseeable future. And now at core we also have Nissa, returning after a long absence, the bigoted Elf now into Elementals and land as her ability set. Green has a few preliminary personality and mechanical bases covered, in other words - A healer who indiscriminately mends friend and foe for the sake of life itself and a mysterious Morrigan-like figure who assigns quests and holds destiny in her hands would go deeper into green.