Dragon's Maze is still many months away from even the first of its previews, but regardless, we have enough information about the third set in Return to Ravnica block to make educated guesses about it. When the details of the Dragon's Maze prereleases were recently made public, we learned an interesting change from the previous two sets: While Return to Ravnica and Gatecrash had both featured five prerelease promos, one per guild, which were allowable in decks during the tournaments themselves, Dragon's Maze is going back to the old model. This means that there is only one prerelease card and it can't be used in prerelease deckbuilding.
This has been cause for a lot of guessing in and of itself. First of all, since there is only one, the prerelease card is likely going to be a mythic rare rather than the "mere" rares that the previous two sets' were. Second, for fairness' sake, it is assuredly not going to be guild-affiliated.
That leaves a lot open to interpretation, but still rules out all of the cards whose identities we already know: Ral Zarek, the blue-red planeswalker, for being for-Izzet-use-only; the Implicit Maze, for being too important to the story, and the fact that it's seemingly deliberately being kept very, very rare by way of its special distribution in boosters; and Render Silent, for already being the Buy-a-Box promo.
In fact, while it's technically possible that the prerelease promo might be monocolored, I say it's ultimately more likely that it's an artifact, so that players with strong feelings about any particular guild can then use it in their guild-based Standard Constructed decks after the tournament.
Here's my best guess:
Pendulum of Hatred 5
5, Tap: Each player chooses a color, then each player sacrifices a permanent of each chosen color. Each player who sacrifices no permanents in this way loses 5 life.
What do you think the Dragon's Maze prerelease promo will turn out to be? Let me know in the comments.