Idk tbh, I think there's really no purpose for Heihachi to return since he's dead, same with Kazumi. I felt her inclusion was pointless since she makes no canon appearance at all in the current time that T7 is taking place.
I think if Heihachi is included, they should make him like maybe an unlockable bonus character who can be easily obtained from like a challenge or something, it'd just be a wasted character slot for him to appear after his fate from the other game. They should make the launch roster consist of the main story relevant fighters(Kazuya, Jin), characters who tie into the major characters(Miguel, Asuka, Lee, Lars, Alisa) and then the last several can be characters like Lili and I definitely think the Tekken 7 newcomers should return too like Claudio and Katarina, Fahk, and Lidia too
The guests from the last game won't return definitely and I think Kazumi should be canned as well, good design but her fighting style and overall presence isn't needed anymore since there's no purpose to her now that Heihachi is dead. New characters are definitely gonna be a thing so I hope they're gonna add at least 3-4 new guys and maybe a couple of new females