Colin would actually make a fairly plausible replacement for Link... even at the start of the game he was really loyal and kinda reminded me of OoT's Young Link in some ways when I first met him (at least before I learned more about his personality). Still, even as timid as he can be he's proven his loyalty and caring nature can easily over-ride this when the need arises... such as when he risks his life to push Beth to safety. Not all the Links have been perfect... I mean, look at WW Link, the first time we meet him he's snoozing lazily on Aryll's Watchtower, and then later on he doesn't think at all and nearly runs clean off a cliff because he sees his sister is in danger and his heart runs away with him. Not exactly what you'd think of being "Chosen Hero" material. Spirit Tracks Link is much the same, dozing at the start and nearly missing his own ceremony, as is Minish Cap Link among others. This shows that The Hero isn't always ridiculously perceptive and fearless and...perfect, only that he can push those fears aside and get the task done. Colin more than meets that criteria. Another little thing as well... the way Colin is bullied by the other childen in Ordon oddly echoes the way some of the Kokiri shunned OoT Link. Again, kid who gets bullied is not exactly what you'd think of for a Hero, but then look at OoT Link. Yeah.
Also, considering the Sword/Shield he's shown with at the end of TP, and the fact he's grown up admiring both his father and Link, I'd say it's pretty likely he did go on to have his own adventures of some sort after the events of TP (regardless of whether he was/became/ended up being The Hero like Link was or just as a normal explorer/ranger/Hyrulean Knight). Whether or not this would make good game material is debatable, but it would certainly be a refreshing avenue to explore: playing as a different character yet seeing the same Hyrule we explored as Link from a different perspective. After all, what would he do now the dungeons have already been cleared? Would he go back and see the very same puzzles/traps we ourselves remember solving and dismantling? Would the temples have become re-infested with evil in Link's absence? Would he leave Hyrule altogether in search of fresh places to explore? Who would he end up with as a sidekick? Would he be forced out into the world after tragic events befall Ordon? Or does he choose to leave? What would his mission be? Perhaps Hyrule would be in danger once again and he sets out to find Link, The Hero and only one thought capable of destroying this new evil, but in the end find that he's actually the one who ended up saving the kingdom as he did so/during his search? Who knows... it sure is interesting to think about.
That said, I doubt such a game will ever be made... Nintendo don't typically go back and revisit characters/worlds like that. And there are a couple of reasons why I personally don't think Colin would work as the Hero... for one thing, he's still too closely tied to his family, and I doubt Uli would be happy with him galavanting around the kingdom alone risking his life. He also isn't really suitable Bearer-of-Courage material, despite his occasional bravery, and besides, TP Link already holds the Triforce of Courage for that era in time (unless he dies...?) so I don't think he could really "replace" Link/properly fufill the role as the next actual "Chosen Hero" or anything. There's something that still bugs me about him as a character as well, and I have to say I wouldn't really enjoy playing as him much... but, as I pointed out, if said game was made it would surely be plausible and interesting enough to make me take a look :3