Kiryu is great, but as far as Sega characters go, didn't DoA5 devs at one point express a liking for the prospect of adding Ryo Hazuki if they could? He might get precedence, and get a release aligned with Shenmue 3's.
VF's Kagemaru also comes to mind and would fit the DoA ninja theme while paying homage to the VF inspiration.
Kiryu IMO is a better fit for a Tekken guest, in that he's incredibly brutal while also being damn goofy at times, much like that series.
I can only imagine the riot if Tifa made it to DoA6 thanks to the Team Ninja work on Dissidia, but did so before she actually joined the new Dissidia.
Can't say she doesn't have some aesthetic affinities with DoA... but maybe not this early on.
I can't help but think that if DoA was to implement a crossover character into its story like Tekken 7 did with Gouki, Kyo from KoF would work great, since like Kasumi, he's also got a bit of a history with clones, and a pretty big movelist that expanded and changed over the years enough to probably work in a 3D fighting game... Mai was one hell of a proof of concept, so it doesn't seem impossible, at least... well, it wouldn't be if the initial released wasn't declared to be a guest-less thing, understandably...
