Gatecrash's preview weeks have come and gone, and the set is about half spoiled, with the rest of the spoiler soon to come. Each of the guilds is more or less a known quantity (though there are still plenty of guild cards yet to be seen, their playstyles are pretty much set in stone), and we know the identities of both of this set's planeswalkers - a monowhite Gideon and a red-green Domri. So we have a very good picture of how the set will pan out, given that its design mirrors that of Return to Ravnica.
Dragon's Maze, meanwhile, is still an unknown quantity. There are only a couple of solid facts about the third set in the block: One, Ral Zarek will be present as a planeswalker card, and two, all the guilds will be represented. This means all bets are off with regard to color combinations for any other planeswalker who might show up in the set. Certainly, that second planeswalker might be a pre-existing character, or might be someone entirely new. A past article of mine discussed the former possibility; now I'll focus on the latter.
At first blush it might seem likely that if that last planeswalker is indeed multicolored, then he, she, or it will be a two-color combination that we haven't yet seen. This leaves green-white and green-blue as the only real options, but Mark Rosewater has gone on the record as saying:
Planeswalker choice isn’t about box checking. We will get there eventually but it’s not the driving choice behind our decisions.
So if it's not an unused color combination, what would it be? Let's look at what colors each of the block's other planeswalkers are:
Blue: Jace, Ral
Red: Ral, Domri
Green: Domri, Vraska
This block, more than any other, is about color balance. The guild system means that every color combination has to get fair distribution of design goodies. So looking at this chart, it's apparent that both white and black are both one planeswalker short of a completely equitable arrangement. My conclusion? It's likely that Dragon's Maze's second planeswalker will be white-black.
Of course, I'm aware that a planeswalker in that color identity was printed in Dark Ascension in the form of Sorin, Lord of Innistrad. But there's definitely room for another white and black planeswalker character. Some possible concepts for such a character might include a ghostly planeswalker attempting to recover her physical body, a cleric who seeks to unite the angels and demons of the multiverse in a bid for theological takeover, or a crusader tormented by the power of his evil weapon.