If we get a return of Patroklos, Pyrrha, Leixia, Natsu, and Xiba styles, I'd like them to be bonus styles, not added to existing styles.
Leixia's newer aspects felt more like the SoulCalibur III Chinese Blade style to me than Xianghua. I like Xianghua being Xianghua. If we give Leixia's "Xianghua moves" back to Xianghua, and bring some more Chinese Blade style stuff to Leixia's moves, to make a more complete Chinese Blade style, that would make me pretty happy for reasons you can see in my avatar.
Cassandra doesn't need any influence from Patroklos, either. Patroklos was already taking inspiration from Cassandra and making the moves feel a bit more masculine. Cassandra may be a bit of a tomboy but she's still got her feminine charms with her moveset and certainly doesn't need to start spouting any nonsense about justice. If anything, I'd like to see Patroklos's moves go to Aeon instead, so he's not so much of a Sophitia clone if they're going back to him having the Xi Sword & Game Shield since they're doing SoulCalibur I base.
Seong Mi-na already has had her own identity in SoulCalibur III-IV and doesn't need anything from Xiba. I was disappointed in Xiba's moves with a Halberd for my Seong Mi-na custom character in SoulCalibur V because it really felt wrong for her. And I already addressed Aeon above.
As far as stuff I'd actually repurpose/keep, I'd like to see the return of Quick Battle, it's a very fun mode and seeing the different custom characters that they have put into the game already is useful to possibly inspire ideas and get the creative juices flowing. Ideally, we'd actually see the return of Weapon Master / Chronicles of the Sword that also implements a wide variety of custom characters, but if that's too lofty a goal, Quick Battle at least is very nice a mode. An expansion I'd like to see, though, would be the ability to use not only their titles but also their templates in custom creation once you've defeated them.