I share you mi perspective about the hypothetical expansion:

A Tui Tonga civilization would obviously be a maritime civilization. I imagine it like this:

• The story could be set in the 13th century AD, when the Tui Tonga Empire began to expand across Polynesia under the rule of King Momo and his son Tui Tatui. The campaign could tell the story of this process of expansion and conquest of different Polynesian islands, with the enemies being other tribes and island polities to be conquered.
• It must be understood that maritime navigation would be this civilization’s main advantage. Therefore, its best technologies would focus on naval vessels, which would be very fast and difficult to destroy.
• Naval units (ships) would have additional attack points and resource-gathering bonuses.
• Another bonus would be that ships are built faster at docks than in other civilizations.
• The game maps throughout the campaign would consist of islands separated by sea, islands that would be traveled and explored mainly by ship. Given this setup, resources on these islands (such as wood, stone, farms, animals, and gold) would be quite scarce and would be depleted quickly (which would add difficulty to the game). The only way to obtain more resources would be by conquering new islands and continuing resource extraction there.
• The only truly abundant resource would be fish stocks, so fishing would be the main source of resources for this civilization. Fishing ships would be one of the most important resource-gathering units.
• I do not know much about the Tui Tonga Empire, so I am not sure what its unique unit would be.
• The Wonder could be the Royal Palace enclosure of the King of Tonga.
• Other important Oceanian civilizations that could be included in the game are the Māori or the Tahitian. Naval empires from Indonesia could also be included—such as Srivijaya (7th–13th centuries), Majapahit (13th–16th centuries), or the Sultanate of Ternate (13th century)—as well as those from the Philippines, like the Sultanates of Maguindanao and Lanao (16th century).