It's hard to deny that a significant fraction of a color's appeal depends on its signature planeswalker characters. when there is a diverse cast of heroes, villains, and those with other motives altogether, planeswalkers can represent the manifold interpretations of a color's driving philosophy. But when this is lacking, the color suffers. Color combinations without sufficiently compelling characters representing them get too often reduced to stereotypes in players' minds, and decks using those colors that would benefit from planeswalkers are hurt by a lack of choice.
This is, of course, easily fixable in set design - but that's for R&D to decide. The best we can do is lay out this wish list of the ones that need the most help:
Monogreen: I am a fan of Garruk. He is cunning, implacable, overly driven by instinct, uncivilized, hates pretense and artifice - all good things for a "nature hero" type of character, and all facets of green. His summoning magic is also something that many players like. But he shouldn't have to carry green planeswalkerdom alone. His only meager assistance in that department comes from Nissa, who is a pretty reprehensible character and got a poorly-designed and linear card in her only version. Green could use, say, a Treefolk planeswalker, who represents the patient and wise side of green - or, better yet, rehabilitate Nissa by remaking her into a real, effective villain instead of one who merely causes catastrophes without knowing what she's doing, and by giving her a card that doesn't depend on running a specific other card.
Red-white: Ajani Vengeant might not be the strongest planeswalker in terms of raw power level, but he holds his own, especially with that pseudo-Lightning Helix ability. His design is a nicely balanced mesh of both red and white elements. Here is the problem: Ajani's gotten the anger out. "Vengeant" no longer describes his personality - and he's no longer going to get red and white card versions for the foreseeable future. I may be biased, as this duo is one of my favorite color pairs, but I feel a little cheated that Gideon didn't discover the fire in himself when he joined up with the Boros Legion during Return to Ravnica block, for example (which would - bonus! - bring him closer to his beloved Chandra). We deserve an extra-cool red and white 'walker in the coming blocks - such as a Ravnican Minotaur who specializes in siege warfare, or a wielder of desert and sand magic.
White-blue: Venser's dead and he's not coming back. His planeswalker card was awesome, but he is not getting any more. One can see how this presents a problem for this color combination. There were hints early on that Jace would at some point be drawn into either white or black magic, but this has yet to materialize, so why not an entirely new character? Venser's "teleportation mage" schtick was a nontraditional white-blue thing, so this time around, we should perhaps see something more familiar, like a lawmage, maybe even literally Azorius.