Imo it's more dramatic, you fight with the 4 leaders, but the ultimate master mind is revealed, the 5th leader and it's someone we know, he's not just another head honcho, a phantom figure at the heart of the CYM. Now I imagine if there was going to be a secret identity, it'll be someone you underestimate, someone who looks weak like Zhu, they don't sit on a throne, but a wheel chair. If Tentei is the ultimate enemy, then you would think he would have retroactive influence on events in the previous games. I think Tentei being Zhu makes "more sense" than a new character, and potentially answers a few questions.
Ren says about Zhu "He used to be the big boss of Hong Kong, I heard he lost it all though".. yet Zhu has this elaborate network of agents, he seems pretty equipped for someone "who lost it all". Yeah.. Zhu has access to a very capable cell of agents, and spies. I guess Zhang leads one front of this cell, and the other front is Tentei.
If Lan Di turns on the CYM, there could be a flashback scene, where he explains to Ryo who Tentei is... in Kowloon, Lan Di arrives at the top to pick up Zhu. Lan Di could have landed on the building, he could defeated them all there and retrieved Zhu, but he nods at the CYM, and signals they should leave, Dou Niu says "This whole city will belong to me" but he becomes frustrated with Lan Di who is implied to have abandoned their deal, and he leaves without Zhu, why? Lan Di may have realised Zhu was a persona of Tentei, Zhu is standing with Ryo's friends at the time. Tentei could have been his mentor in the CYM, and played cruel games on Lan Di as a young man, in an attempt to corrupt him. He is thinking "I'm out of here, not another game.." Repeating myself.. perhaps Lan Di has own agenda to focus on, related to his father, and to contrast with Ryo following the footsteps of his father, and visiting the same places/people.
5 star corp maybe was a shadow company owned by the CYM, so Zhu could present an unsuspecting backstory for this persona, a front, and then he places himself in Ryo's parameter, (and perhaps also in Sunming's "his best friend") in some long manipulative game.
In the DLC for 3, I think it would be better in retrospective with this reveal, Zhang reintroduces himself "I heard the CYM are in this area, I came by this information by Zhu" that line is absolutely delicious if we know Zhu is the true leader. The guy trying to betray Zhang says "I won't be satisfied, I want to be like Zhu, I'm not like you (Zhang) I won't be satisfied unless I'm on top" ok that sounds like classic foreshadowing if Tentei is literally on top of the CYM and everything that is going on. But again Ren said Zhu "lost it all" something sounds sketchy and not adding up here. Then later Zhang says "this is my organisation's problem" he doesn't say who his organisation is, which is shady. I don't think they're the CYM per say, but I believe they're an extension. Then at the end of the DLC, Zhang says Zhu will give him a reward in Hong Kong next time he's in the area, Yu Suzuki or the writers could focus on new characters in further game if the series contintue (e.g. Ziming or 2 other leaders) it may have been cleaner to just end that plot there, but he gave the DLC an intersection into the further story, and I guess that's because Zhu still has a noticeable role to play in the series. He told Ryo a lot near the climax of II, but I believe his story might not be over yet.
Speculating further, the leaders of the organisation may be after different things.
Lan Di = Wants the treasures, he has 'sold his soul' to the CYM, like Xiuying described Ziming. He endured Tentei's training/tests, but he's stlll not a complete tool, and has his own code,"I'll allow you to die like a warrior" also in comparison to Sun who is just chaos and would rather set your base on fire. I think Lan Di has his own agenda, he wants the treasure, for the same reason his father became enticed by it.
Niao Sun = I think she's jealous of Lan Di and believes he was considered the favourite, acting leader of the CYM. "The CYM belong to me now" she wanted to become the favourite, and now she may want the treasure, to amass enough wealth, so she can burn the old organization down, and rebuild it her own image. This fits her old whole red phoenix motif.
Tentei/Zhu, wants to engineer a battle between Ryo and Lan Di, and begin an ethereal conflict involving Shehua's power, .. war representing Chi You. Doesn't really have a personal motive like Sun and Lan Di, he's top of the food chain and wants to screw everyone else over. The most evil of the evil, Palpatine-type.