Is that running with the assumption that every playable character needs to also appear on the story mode timeline?
Not every character, but again, if Geralt, a guest character - who will never appear in another Soul Calibur game again - is getting a story mode; it is best to assume that every returning character is going to get one too. THose that wouldn't get a story mode, are likely to be boss characters, like Inferno. It's reasonable to assume that one of the newcomers could be a boss character too. Why they wouldn't get a story mode would be subject what's in the story that would prevent them from getting one.
As to whether or not, more slots would appear in the Story Mode after completing the game or after completing certain character stories, that's not really a good question. Because if they could hide slots in the story mode? Why would Namco publish a screenshot of the Story Mode with 20 character slots with several of them blurred out? If they had a Story Mode that could hide character slots, surely, they would've published a screenshot that hide those slots.
Likewise, given the character select screen has a limited number of slots, and one of those slots is taken by a random select, and we know that character creation is coming back, so another slot on the select screen would have to be taken for CAS, we can see that the number of character slots is about 22. There are 20 slots in the Story Mode. Therefore, it is reasonable assume that the remaining slots NOT in the Story Mode are reserved for boss characters.
It is certainly possible for them to reveal additional story mode character slots. Maybe those two remaining slots are boss characters and they too could have a story mode. It's possible that there may be three boss characters for one hidden slot in the middle (though I suspect the DLC slot is there, if not on the lower left corner). But this is speculation, and I don't have enough evidence to justify this possibility. As it stands, based on what we know, three of the six story mode slots have been revealed; that leaves three left. We know Cervantes is probably coming and will fit somewhere on the Story Mode (I think we all have a fairly good guess which slot he takes); thus only two slots remain on the story mode and for returning characters.
If not for returning characters; then probably one more for a returner and one more for a newcomer, but for some reason, I don't find this likely. But I don't have any evidence to justify why, only that Algol in SC4 was a newcomer and a boss character, but he doesn't really give enough credence for that.