YOU
The biggest problem would be explaining the Multiverse to the uninitiated. They've got to cover mana, spellcasting, planes, and Planeswalking. That could potentially be quite a bit to fit in, which could bog down the story. Unless they decide to just ignore all that Planeswalker stuff =_=
Actually, just don't explain mana and you're pretty much set. The opening monologue just needs to be something along the lines of, "Across the many planes of existence, there are special wizards, known as planeswalkers, who have the ability to step from one dimension to another at will. Their existence is a tightly guarded secret, even as their actions affect all around them..."
Then just do a cold open of Jace and What-His-Face from Agents running from a bunch of pissed off guards, loot in hand. Jace waves his hand and a wall of ice seals the corridor as they duck around the corner. There's a flash of light, a Stargate-wormhole SFX, and then they come to a rolling stop in a dark, garbage strewn alley. What's-His-Face makes a pithy comment about Jace's landing skills as he picks a banana peel or other silly bit of refuse off of Jace's head, before helping him up where they head off to a bar to meet Lilliana. Takes maybe five to ten minutes and you've show magic, planeswalking, and explained why our MC is special little boy enough for the plot to be following him.
Actually, could the same thing with Dack, which might make for a more interesting movie if they tried to adapt the comics instead of one of the novels.