This would be a one-shot BIONICLE set of the Mata Nui "Matoran Universe" robot. This was the "true" form of Mata Nui and was in fact the giant construct that much of the story took place in. It was only ever seen in control of the Makuta Teridax and was destroyed not long after by Mata Nui in another gigantic body, but it still remains an important part of the story and was for a long time the primary mystery of the entire plot. Of all characters we never got to see released due to BIONICLE's end, this is one that I have seen fans wish had been made as a set more than any other. While the line itself can't be so easily brought back, a single set - now that, I think, is possible.
This would actually be able to be built rather easily with the Hero Factory modular system, as it does not follow the overly tech-like style of old BIONICLE sets and is much more streamlined. So it would fit in with currently-released sets too. The model here simply serves as a proof of concept - if anybody else thinks they could make a better one, by all means go ahead (just try to keep it relatively simple).