In terms of planeswalker design space, there's a lot that's yet to be explored. For example, the two-color combinations are certainly hardly exhausted, and green-white doesn't even have one planeswalker to its name yet. And the only more-than-two-color planeswalker printed so far has been Nicol Bolas, Planeswalker. Now, Bolas had to be the color combination he is simply because of his character's history - but there's no rule that says only Elder Dragons get to wield three colors of mana at once.
When mrmond4y asked Mark Rosewater on his Tumblr blog, "Do you think we'll see more tricolored planeswalkers, or is that something reserved for Bolas ('cause he's Bolas)?" he answered:
I’m sure we’ll make more but it’s something that will have to happen organically.
"Organic design" is something Rosewater likes to emphasize for all planeswalkers, as opposed to "box-checking," which is why all of the bicolor combinations haven't gotten one yet, among other things.
As for who makes sense to get a tricolor new version? Sarkhan might get his green back and be the first black-green-red 'walker after finally having tapped into the true essence of Dragons, and an undead Venser (yes, we can dream) would likely have to be white-blue-black.