If there were no limitations at all, I would have a conclusion to the serials UCS, ultra-detailed versions of the Toa Mata.
But the truth is, if we’re going to get a “UCS” set of any kind, there would only be one. Any kind of multi-set “line” would probably be aimed at kids. For Lego to put the effort into a “adult/collector’s” anniversary, they would dump everything into one set. (Or BrickHeadz)
And that’s why I think the Great Spirit Robot is such a great candidate. It’s a single model, and a massive, iconic part of the Bionicle story that never got any kind of set depiction. Additionally, it wouldn’t require any crazy new molds; it could be done entirely with System. Just look at that Mata Nui bust that I posted here a few days ago:
If you showed me a picture of that without context, I would it assume it was an official 20th Anniversary set, and I would be happy.
(Plus, if Lego is worried about sales, Giant Robot With Lasers has mass appeal outside of the Bionicle community.)