I'd like if they polished up Bionicle, tweaked it (FIX THE DARN CRACKED JOINTS), simplified the story, just a little, so that newer fans could jump right in without having to read everything that came before. And then give it a better ending (in my opinion, the ending just didn't feel epic enough to compliment such an epic story-line). I don't think its return is likely, but then again, that could just be the nagging little voice in the back of my head.
Hero Factory and *insert name of new theme here* could both run at the same time, Hero Factory staying how it is, to appeal to the "younger" audience, and *insert name of new theme here* could lean more to the dark, gritty, mature side of things. On a common sense side of things, I think it'd be the optimal decision, but then looking at it from the business perspective, I don't think that's gonna happen.
Above all, I hope that it'll be the line we've been waiting for since 2010. With a real story-line, real characters and a real setting.