I'd prefer it if they gave more focus on putting non-ninjas into crossover games and spinoffs so that they can have exclusive DLC stuff. They are very much left out and secluded on their own series honestly.
Bayman's 5/5U character model fits well most 3rd person shooters so it shouldn't be hard putting him in crossovers like Resident Evil, Metal Gear Solid, etc. By DOA6, he could finally have some exclusive DLC of him in a BSAA uniform.
Ninjas are probably the biggest selling point sadly enough. Let's look at character potential here for non-ninjas.
Bayman and Christie are assassins, opens up tons of stealth driven/FPS/etc possibilities for a serious story revolving around those characters, maybe even Leon as I believe he was a former assassin and you can get more in depth on the death of his wife. Most likely M rated games with some blood for realism but you've got a great deal of potential with these 3.
Ein... eh, could go in depth but considering at the end of the road he's Hayate again it ties back into ninjas so, pass.
Leifang - college student and kung fu fighter following around generic Bruce Lee homage character. Aside from some possible backstory and a hands on look on how he saved her, not really anything to make a game or spinoff of.
Jann Lee - lol no.
Gen Fu - Not popular or relevant enough to warrant a game, an old man running around collecting money to save his sick grandaughter is an interesting one but not really something you can make a game out of.
Eliot - Unpopular, not really relevant either, him or his journey, pass.
Lisa - Ties into ninja related story. Nope.
Kokoro - Decently popular and tied to Helena but nothing warranting her own spinoff, best bet is an appearance in a game where another char is the focus.
Hitomi - lolno, boring. Her life was most likely largely a snore before meeting Ein and post DOA5 she's probably moping around about losing to Jann and that Hayate will never return her love, pass.
Brad - Potentially a game centered around him trying to find whatever he was looking for in DOA3 prior to the game itself, but not popular enough of a character to carry a game.
Mila - Regular person, snorefest, pass.
Rig - Villain character, tied into ninja related story, not even a candidate for a spinoff.
Bass - We have enough wrestling games, a motocross esque game maybe but it might as well just be a minigame of some sort, the guy's not popular enough to carry a full game anyway.
Tina - Plenty popular but nothing interesting enough to make a game out of, not a serious one. Too built around fanservice also, you might as well just create a DOAX3 and make her the protagonist if you're going to make a game centered around her.
Zack - Would probably be some Japanese crazy nonsense game, pass. But if we can get a more interesting look on his backstory and maybe a reason why he acts the way he acts and how he came to be who he is, I could see it. Can't see a game with Zack as the main man tho.
Helena - Huge potential for her own spinoff but I'm stumped on what the gameplay would consist off. Shooter? no, beat em up? lol no, action/adventure? She's a rich company head why is she running around? She's got story potential but no real gameplay style suits her character, she's more of the important NPC type of character.
That leaves Bayman, Christie, Leon and the ninjas. Might as well use the ninjas.