That is most definitely my largest concern when it comes to BIONICLE becoming a System theme. Not because it wouldn't work, because as @TeslaEffect notes, BIONICLE has a history of well-built and intricate vehicles and technic-oriented creature designs that are among some of the theme's strongest sets. That said, BIONICLE's core is not those sets, but rather its character-based builds. Having a theme mainly comprised of Battle Vehicles would not necessarily be bad, but it would make for a different experience than what we're used to.
...THAT BEING SAID
I think that, as it stands right now, BIONICLE returning as a System theme would offer it the greatest chance of being done "properly." While the core of BIONICLE is its constraction-based builds, I feel that the TRUE spirit of the theme is the feeling it evokes, the aesthetic design, and the unique style of the world being a mix of tribal and technological builds. If BIONICLE came back as a System theme but featured a full-scale TV series since it wasn't being held down by constraction, a ton of cool, detailed minifigures, some excellent location-based sets which focused on BIONICLE's excellent history of worldbuilding, and yes, some snazzy Battle Vehicles along the way, I think it would be worth it. Throw in some constraction characters too, if the budget permits, and we're just golden.
BIONICLE is about a heck of a lot more than the sets. If the story was allowed to flourish, the world permitted to grow, and the marketing budget expanded to allow the theme to reach a wider audience, I don't really care what style of sets we receive.