Each year, Wizards of the Coast releases two Duel Decks products. One of them pits faction against faction, such as Phyrexia vs. the Coalition or Izzet vs. Golgari, and the other has two opposing planeswalkers, such as Venser vs. Koth or Sorin vs. Tibalt. These all tend to tie into the regular Magic sets they share an environment with somehow: The faction decks have to do with the fall set that they precede, and the planeswalker decks are themed around the previous year's planeswalkers. Since we know that the next faction product will be Heroes vs. Monsters, the next thing on the Duel Decks agenda will be planeswalkers. But which ones from the Return to Ravnica block?
The ones that are available are Jace, Architect of Thought, Vraska the Unseen, Gideon, Champion of Justice, Domri Rade, and Ral Zarek. Now, Jace has already been in a Duel Deck - in Jace vs. Chandra - so it's not likely he'll get the spotlight again. But Gideon is an incredibly popular character, and it's probably due time for him to get a Duel Deck of his own, even if his new version is less beloved than Gideon Jura. So we can practically count on him to be one of the featured planeswalkers.
As for his opponent? Domri Rade is also a favorite planeswalker, for his card (which rates high) if not for his character (which most players seem to think is obnoxious from the little we've seen of him). But Heroes vs. Monsters is a red-white deck versus a red-green deck, and with Gideon firmly entrenched in white and leaning red, R&D would likely not want to duplicate the color combinations of the other Duel Decks released this year. That leaves Vraska the Unseen and Ral Zarek, both native Ravnican planeswalkers, as contenders. Of those two, Vraska has a controlling, slowly destructive playstyle, building up into a win that's accomplished via a special creature - resembling Gideon's style very much, even too much. Ral, meanwhile, is erratic, manipulative, and rather faster and more psychologically confusing and crazy than Gideon, so he'd make for a more interesting matchup.
In other words? Look for Duel Decks: Gideon vs. Ral to likely be on store shelves later this year, with Duel Decks: Gideon vs. Vraska with longer odds.