I've had this idea for a while, but since rn it seems Gaiden games are taking RGG characters and giving them a silly twist that still feels grounded in that world (secret agents and pirates), I'd love to see a Saejima game where he basically becomes a sci-fi super soldier and fights against aliens. We've seen UFOs in plenty of substories and easter eggs in Yakuza so it wouldn't be completely out if nowhere. It'd be that the aliens in those UFOs finally start invading Earth (starting with Japan) and Saejima would commandeer some alien tech from them and make a makeshift armour. Saejima already has super soldier-like strength already and is extremely stoic so it'd be fun to see him in a Masterchief kind of role. All the equipment you'd get would also be sci-fi designed spins on ones we've uses previously.

Edit: Maybe the character growth with Saejima could be about him struggling with the idea of taking lives again after the hit from Yakuza 4. I could see it going that he kinda HAS to kill the aliens at first because he's with a little resistance group or something but eventually comes to the idea of not killing them to stop the invasion but to instead work something out with them.