This is an intriguing rumor, but I'm highly skeptical. Zelda has gone on in excess of two decades, and in that time there hasn't been a single title where the main character wasn't Link. I mean, we have Tingle's Rosy Rupeeland, sure, but that can hardly be called a Zelda game. In fact, I can only think of two characters you ever directly control in the series aside from Link himself (and whatever he's riding)-- Kafei in Sakon's Hideout, and the seagulls in The Wind Waker. Both are necessary for completing the game 100%, but neither are crucial to the plot as a whole. I'm willing to bet there are a few more that I can't remember right now, but The Legend of Zelda is first and foremost all about Link.
That said, if they were indeed intending to create such a massive paradigm shift in the series as this rumor suggests, Colin would be an excellent choice. He knows Link personally and was so inspired by him that he tried his own hand at heroism, and did a pretty good job, as ZV pointed out. He has the seemingly-required blonde hair for the job, and is good enough friends with Link that if/when Link decides to step out of the limelight, it wouldn't too much of a stretch to imagine Link passing on his hero's clothes and weaponry to Colin. The kid definitely has heart, and a hero's resolve.
Now, if Colin were to become the new hero of Hyrule, who would the antagonist be? In TP, Link not only defeated Zant, but also chased off the moblin horde, cleansed the Arbiter's Grounds and other dungeons of their evil inhabitants, and pretty much restored the land to its original state. They'd have to bring a completely new villain into the mix. I mean, I suppose they already did that with Ghirahim, but I still really know nothing about Skyward Sword (I know you're disappointed, Mags :<). Vaati is unlikely, and his story has pretty much been covered from beginning to end. Bellum is unlikely as he has never been mentioned outside of PH. Dethl, Majora, Onox and Veran are also contained within their own worlds.
Essentially, all of the main evils in the series would either not have a good reason to appear in Hyrule, or would have already been dealt with by our main man in green by this point. So who would the villain be? I suppose through process of elimination, we're left with the possibility of Dark Link, or the dark magi responsible for the Fused Shadows. Either would be quite interesting, as we've already seen hints of them in the visions Lanayru gave Link which sets things up quite nicely, and they seem to be the one big plot thread that Twilight Princess didn't delve into.
I dunno, I guess I'm just feeding the fires of rumor at this point, but it's quite the intriguing concept. Thoughts on my probably-horribly-flawed breakdown?