When you really think about it, the Tekken characters fit here better than most Fighting Game characters on the roster. After all, their world has demons, dinosaurs and robots, making it very possible for them to come out on top against these curiosities. Hell, the Final Boss of Tekken 6 is an Egyptian god, of all things!
The Color-Coded for Your Convenience intro of the sequel, where some of the party members have a different glow before the opening ends, are thoroughly justified when you consider how each behave from their respective video games.
Kazuma Kiryu and Goro Majima are often paired with Phoenix Wright and Maya Fey in promotional materials and within the story of Brave New World (Phoenix gets Majima acquitted of his actions from Yakuza 4 and the latter is indebted). Both series have had film adaptations of their first games directed by Takashi Miike.
Adding to this, Hiroki Narimiya, the actor who voices Masayoshi Tanimura (one of the protagonists of Yakuza 4), portrays Phoenix in the aforementioned movie adaptation.
Ryu has a Running Gag of suggesting people perform a "Tatsumaki Senpu Kyaku" while in mid-air if the party finds itself performing aerial maneuvers. This may be Played for Laughs, but recall in the first game, Ichiro ate himself up when he thought some of the cast died from a large fall, unaware that most of them, Ryu and Ken included, had ways to control their movement in the air. Ryu's suggestions to perform the technique is his way of ensuring the party would not be placed in the same situation as before.
Xiaomu ragging on Sheath about the "Japanese [she] picked up from the Internet" in the localization becomes hilarious when you take into account Xiaomu herself is already a walking, talking Shout-Out factory. It's not too far-fetched to believe Xiaomu has already met people like Sheath before at some kind of Anime, Manga, or video game convention.
How can the party easily damage Mechon in the sequel despite the Monado and Anti-Mechon weaponry being the only things capable of leaving more than a dent? That only applies to beings from Bionis, and since the party is comprised of people from other worlds, of course they would be using weapons that aren't made by Homs, which prove ineffective against Mechon.
Mettaurs are defensively strong when players face them: getting to hit them at all can be a chore. However, when Mettaurs attack allied characters and players perform a counter attack, they become much easy to destroy, usually taking no more than one or two moves to finish them off, exactly like how they function in their home series.
Certain characters appear using their non-canon appearances such as Yakuza: Dead Souls and Tekken Tag Tournament 2; they're considered fair game since the series itself is on non-canon territory from the start.
Then there's the fact these non-canon games are alternate universes to the parent series. The plot of the games just happens to make contact with said splinter universes instead of the "main" one.
In the sequel's opening sequence where Reiji and Xiaomu face off against Saya and Sheath, their positions show something of their character traits. Reiji and Xiaomu are back to back, indicating how long they've been working together in Shinra and that they trust each other to have their backs. Saya and Sheath are face to face, indicating that neither really trust each other and the rather backstabbing/doublecrossing nature of Ouma.
In the original Project X Zone, Kite is paired with his R1 deuteragonist Black Rose. While Black Rose does not come back for 2 despite Kite returning, he does get paired with his R2 successor: Haseo.
: Haken is accidentally sent into Sanger's dimension along with him. Since the "Cross Gates" are what bridges both characters' respective universes, and without access to one by the end of The Moon Dwellers, Haken is unable to participate in Brave New World because he's literally Trapped in Another World.