Just like with Final Fantasy, the first game in the Yakuza series (now called Like a Dragon) was intended to be a standalone release. It was therefore a complete experience from start to finish, where everything had to be just right. In an interview with Famitsu (via Automaton), the series' executive producer Masayoshi Yokoyama now reveals that this has become a mindset that has remained with the developer even though it is now a universe of its own.
It was only with Yakuza 3 that he began to see the phenomenon as a series, and even now, when new games are being developed, they are still treated as if they were the last. That's why he hasn't yet thought about what will happen to the new protagonist Ichiban Kasuga after Like A Dragon: Infinite Wealth:
"Even now that it's a series, I still create each installment thinking that it could be the final chapter. As an example, I haven't thought about what happens to Ichiban Kasuga in the future yet."
Although he admits that popular characters have a better chance of staying around longer and getting a bigger role, he says he didn't create Ichiban to be popular, but focused on making a successful game so that popularity could potentially come from that:
"Fundamentally, I believe character popularity won't emerge unless the game sells well. So I've never created a character thinking that they are going to be popular."
Like A Dragon: Infinite Wealth was released almost exactly two years ago, followed by the spinoff Like a Dragon: Pirate Yakuza in Hawaii a year ago. Although Ryu Ga Gotoku Studio is incredibly productive, they are currently also working on Virtua Fighter 6, the remake Yakuza Kiwami 3, and of course they have more games in the pipeline. We don't know when we'll see Kasuga again in a new main series instalment, but it's likely to be no earlier than 2028.
Until then, we can take comfort in the fact that he was released last month in Sonic Racing: Crossworlds as a playable guest character.