They did, and they sort of worked for what they were trying to do, but it doesn't work when only one of six of the Toa looks mildly different in shape or size.
Well, if I were running Bionicle from the start, I wouldn't have gone that far down the sci-fi route.
But if we're strictly talking about how we would improve them as they stand now, I don't think those themes are mutually exclusive with color or a tribal aesthetic.
Two of those themes are already natural elements in Bionicle, water ('07) and air ('08), and the designs for those years could lean heavily towards the aesthetics of the element (think Moana for the Mahri and Avatar's Airbenders for the Phantoka).
You could make the case to keep the dark and grungy style of '06 as it stands and just have that so it stands out (and so the normal aesthetic doesn't get boring). Or, you could make the sort of elemental theme stone, and make the aesthetics really rough, hard, dare I say...
I'm kind of inspired to draw that out now...