What boggles my mind is why this series has given so much importance to the dragon and phoenix symbolism but has mentioned absolutely nothing about tigers and tortoises aside from some Lucky Hit stands and Yuanda Zhu's key puzzle?
The dragon and phoenix symbolism is really iconic to this franchise: Dragon and Phoenix mirrors, Lan Di representing a dragon and Niao Sun representing a phoenix, "A dragon shall emerge from the Earth...a phoenix shall descend from above", "Go with the one who holds the Phoenix". Dragons and phoenixes are EVERYWHERE in this series. But tigers and tortoises? Nothing.
If the Tiger and Tortoise leaders have had their roles reduced or been outright cut, I wonder if this was done to keep this series better focused on the rich dragon and phoenix symbolism. It could feel a bit odd if someone randomly says at this stage: "By the way, Yuanda Zhu and Shenhua's ancient poem are both wrong. There are Tortoise and Tiger Mirrors as well! And we forgot to mention them in the ancient prophecy too. Oops!"
IF the Tiger and Tortoise leaders have indeed been cut, could it be because Niao Sun and Lan Di were the only two most interesting characters of the four bosses? Its rather telling that neither the Tiger nor Tortoise leaders were featured in any '90s-2000s promotional material, but Niao Sun who wasn't due to appear until much later in the series already had her face plastered everywhere before Shenmue 1 hit shelves. Surely, if the other two members were just as interesting/fleshed-out, they too would have been featured alongside Niao Sun to further hype up the Chi You Men?
Maybe their removal will allow Yu to better focus on the dragon and phoenix symbolism without having to suddenly introduce tiger and tortoise symbolism this late in the game. So now, we might just have a Dragon leader (Lan Di), a Phoenix leader (Niao Sun) and a Heavenly Emperor (Tentei) that comprise the higher Chi You Men ranks.
Or I'm wrong, and the next two leaders are still planned. Who knows?