Raansu could be correct. As much as it's good to see more VF greatness, the key members from Sega are not present in DOA6 credits list like how it was in DOA5.
With the current state of the game, Sega probably won't even do it. DOA6 started off on a low budget, and contacting Sega is like trying to find unicorns in a garden.
Actually, finding and contacting Sega is not even an issue. The problem is that Sega has a very specific branding strategy, and won't give out their characters licensing without a contingency for creative ROI agreement. Koei Tecmo also has a model for creating games under a specific set of circumstances, such as cost reduction and employee loyalty investments.
It's DOA. It's not like inconsistency for character inclusion or taking chances has not been a thing. A good example is Mai. According to Shimbori, Mai only returned was because their contract was "still" up with SNK, so they had a window time for such characters to begin whereas VF's expired along with a few of Sega AM2's key members not present in current DOA's credit roll (Junpo Sawawori and the others).
We also have to remember that returning "same" guest characters from a previous series is rare. Same series have returned in crossovers before, but very rare to be the "same" characters. Particularly Jacky where people need to know how he also performed business-wise, especially being included in this game series where the audience and entirety of the game is extremely niche.
I'm almost certain SNK doesn't have a strict character licensing policy compared to Sega. Sega has always been very nitpicky with who and how they work with other companies. They put high priority to not squander any of their well established content, and will go that extra mile to keep it that way.
Yes. I also mentioned many many times in the past that Sega are also very keen and firewall heavy when it comes to their characters which is extra layer of Sega purposely becoming a ghost, however marketing tactics makes shells of itself. But again, it's DOA. Wouldn't be surprised if Sega doesn't do it at all.
When it comes to "contact Sega" it's about the global meme on whether they'd actually do it or not because of multitude reasons. Good Ex is Sakurai via Bayonetta in Smash where he went: "I thought they always change phone number but finally got a hold of them! (laughs)" because Sega has been a meme over the years to be "ghost", Sega wholeheartedly agreed with it but in the end it's not easy.
Although, I honestly wouldn't mind seeing more SNK characters make their way into DOA.
If TN could make it happen again with a range of SNK characters, the way they did with VF, then DOA will raise a remarkable awareness for being a fighter crossover friendly title, and that could be big enough to make DOA one of the most iconic fighting games in the near future.
...And then they rerelease DOA5LR and DOA6X bundle pack, and have two online active servers for VF and SNK lovers...Ooo, that would be too cool of a move from TN