Let's be clear: Greg's not the problem here, or at least, not the main one. The fans are.
They're the ones who decided, after Bionicle's end, to consider Greg the sole arbiter of canon. During Bionicle's run he may have been the "mouthpiece" of the Bionicle story, but he was only one member of a team. And that team, for the most part, did a great job of keeping the overall story on-track. Back in those days, there were questions that Greg DECLINED to answer simply because it was not his position to do so—some decisions could only be made with the input of the story team, to avoid conflicting with the overall thrust of the story. There were checks and balances. All that went out the window when the theme ended, the story team disbanded, and fans decided that they would rather let Greg string the story along indefinitely than consider it a closed book.
And now, they're the ones badgering Greg to canonize every single ambiguous detail from the Bionicle story. So far, at least from what I've heard (and I lost interest in the minutiae of the classic Bionicle story long ago), it's not a case of him canonizing terrible suggestions. The problem is that he's canonizing suggestions that never needed to be made. The Bionicle story isn't any better for knowing so-and-so's mask, or weapon, or what this character or that would look like as a set. If anything, it's worse—fans are continually eating away at all the mysteries in a story that was characterized by mystery, and the little details that used to be open to actual creative interpretation in a brand that was intended to allow for creative play. Let's be clear: every time Greg canonizes a character's mask, it DECANONIZES every other possible mask that character could have worn. Every time he canonizes a weapon, it DECANONIZES every other weapon. To petition Greg to include your headcanon in the story is to exclude every other fan's interpretation of the story.
Now, some may argue that this doesn't matter. If the canon disagrees with you, just ignore it and come up with your own headcanon! But there are problems with that idea. Firstly, the organized Bionicle community has an obsession with canon. It's why we have to deal with these idiotic suggestions in the first place—a lot of Bionicle fans CARE about the actual canon, and it leads them to want to make a mark on it. Secondly, the fans that don't like suggestions? They DO ignore it! They do like I did years ago and divorce themselves altogether from the canon—but that, of course, means divorcing themselves from huge parts of the community that they used to enjoy. I used to love coming up with theories in S&T, because it was neat to try to figure out how the Bionicle universe worked, or try to predict the twists and turns of the story. But now I can hardly follow the discussions there, because discussion of the story often relies on a shared interpretation of the story—a singular canon—and the more of that canon you ignore, the less common ground you have with other fans to base discussions on.
I'm very glad that Bionicle is being rebooted next year. The old story, at this point, is a lost cause. It should have been ended definitively years ago, but fans opted to keep it alive long after the point of death, and now I hardly get any joy from it. Bionicle 2015 is a much needed fresh start, and I dearly hope that it can learn from the original theme's ample mistakes. And the biggest mistake I want it to avoid is letting the inmates run the asylum, as the original theme did.