Brave/WMG

The main conflict of the film does not result from Merida's wish
It's the fact that she makes it in the first place. The "sacred custom" she defies is not leaving magic to The Fair Folk. When things go to hell, the people blame the royal family for the mess and turn against them.

The Witch will help Merida
In one of the trailers we see things that look like will-o-the-wisp. Merida will make her "bad wish" on these, and the Witch will help Merida fix it.

Merida will not marry/have a Love Interest
At some point she'll befriend her former suitors and become One of the Boys. She will not, however, get together with anyone.
 * This theory has been officially confirmed.

Merida's parents had a Perfectly Arranged Marriage
In medieval Scotland (hell, in any medieval location) royal marriages were usually a political arrangement. Merida's mother Elinor was likely a noblewoman from a powerful family who was set up in an Arranged Marriage with Fergus when he was either a prince or already a king. Opposites Attract ensued, and the two grew to love each other. They had a daughter and three sons, so their compatibility must have been good. This will likely be brought up when Elinor is pressuring Merida to choose a husband. "You and X will learn to love each other eventually. Your father and I did..."
 * I like this theory. It gives more weight to Merida's question in the trailer "If you had the chance to change your fate, would you?", especially if Elinor is the one she's talking to.

Mordu is connected to the story of the fallen prince
In the Japanese trailer, Elinor tells a story about a prince who used magic that destroyed his kingdom. Later, there are shots of Merida exploring a ruined castle and broken tomb with deep claw marks. She is then attacked by a black bear who looks a lot like Mordu, who is apparently using the ruins as his den. So Mordu could be a victim of the fallen prince's magic, or maybe even the prince himself.

Merida is the heir to the throne because...
...of her little brothers. They're triplets. Despite the fact that they are boys and should technically be in line before Merida, it would cause complications to leave the kingdom to only one of them. The kingdom would be divided by jealous fights between the three (grown) brothers and the public fighting over which brother is best fit to rule. Civil war would probably ensue. So the throne goes to Merida, who (despite being a woman living in medieval times) is already grown, quite smart, and has the capability to lead. In the Japanese trailer it's hinted by Elinor that Merida is going to be Queen someday. Also, there's the fact that Word of God has said that Merida's the only one who can tell the triplets apart, so that would also naturally cause problems.
 * This is still Medieval times, though, so even if Merida is the eldest and most qualified to inherit the throne, she'd still be required to get married and produce another worthy heir. Which makes the whole business with the suitors all the more important, and Merida's rejection of them all the more problematic.
 * Long story short: the Scottish royal dynasty is screwed.

One of Merida's suitors will actually like her
He'll become the Abhorrent Admirer and will be a source of comic relief as Merida repeatedly rebuffs his advances.

Ending theory

 * Or, in a particularly tragic Downer Ending,
 * Both theories are jossed,

Elinor will become a Mama Bear.
Because it would be awesome, especially if she learns anything from

Merida will become a Politically-Active Princess.
She and Elinor will both learn An Aesop after their adventures and reach a compromise; Elinor promises not to force Merida into an Arranged Marriage, and Merida promises to take her training to become the future queen more seriously. Also, Elinor will let Merida remain a Badass Princess but Merida will no longer be a Rebellious Princess because she'll have matured enough to understand that being a good queen means putting the needs of the people before her own.

Merida is secretly the alternate universe lovechild of Wally West and Artemis Crock or Clint Barton and Natasha Romanoff.
They're both redhead + archer pairings, see.

Calvin is a descendant of Clan Dingwall.
Come on. Look at Lord Dingwall's son. Does he look like anyone you know?
 * The hairstlye, kinda. That's about it.