This seemed too speculation-y for S&T, so I'm throwing it here.
Greg has stated he knows for a fact what Takanuva's destiny is, but that he'd only reveal if it was relevant. This seems to indicate that it was something developed by the story team early on in Takanuva's existence, completely independent and unaffected from the canon that came after.
And, well...the end of an Av-Matoran's life cycle is to become a bohrok, that was a fact that was known to the story team since the creation of the bohrok...and aren't the bahrag just big bohrok? the answer is no, of course they are not, but you understand why my logic may have ended up there.
I dunno...The fact that Greg would know Takanuva's destiny way before it would even be useful to have decided such information always seemed odd to me. Thoughts?
EDIT: since I wrote that, I now know Greg revealed that Takanuva was supposed to play a crucial role in the civil war with the great beings, and I realize everything I wrote is wrong. Eh. I still like the idea of Takanuva becoming a bahrag, so I won't delete this.