The Dark Hunters were always one of my favorite villains/organizations in all of G1. If they could somehow be in the G3 it would be awesome. My idea on how it would go would be:
There is another island away from Mata Nui with its own Mask Maker that mainly benefited off of industry and stuff. It was very polluted and had harsh working conditions. The mask maker was also kind of incompetent and let crime and corruption go untamed. One day, a captain of industry(the shadowed one?) who was known for manufacturing weapons and robotics to sell to different armies decides he wants to be the leader of the island and dethrones the mask maker. After this the industrialist starts commissioning tons of weaponry and machines to be built and decides to rent them off to people as a private huge army.They are known to be unscrupulous(of course) and, when the client refuses to pay, they use the machines he rented out against them and take over their land. This has led to them becoming a small empire.
They could factor into the main story in several ways: a) They are in possession of one of the legendary masks as it was given to them as payment, b) Makuta buys stuff from them to use in his army, and/or c) Makuta buys stuff and refuses to pay, causing them to invade Mata Nui.
They would also have elite mercenaries as part of their business so you could stick with all the cool unique characters we got from the Dark Hunters.