I'm sure that the only ones to be cut will be the Virtua Fighter ones (unless they will not decide to renw the collaboration with Sega, but very likely we will get other guest fighters) and Alpha-152 with good chances.
I consider all the old characters until DoA4 - excluding the bosses - part of the regular cast, so I don't see Ein or Leon being cut, overall the latter considering how much TN worked on his fighting style.
With the exception of Phase 4 that has a role even in the story mode of DoA5, all the other original DLC characters were presented in the story with background hinting to relationships or past events, in their description texts as well as in the game with their tag interactions. Raidou was presented with an huge redesign and as a character returned in the story with a plot twist, affiliated with Donovan and having lost his memory; Honoka with a secret behind her power source; Nyotengu with a not explained reason about her presence between the earthlings and overall toward Hayabusa; Marie Rose with a secret about her real persona and role - hinted being tied to Helena and Doatec. It's important to think how TN retconneced the description that they used initially for Nyotengu, so apparently this should mean that they are giving really importance to these few infos that they wrote for us: they seems to be not just a text for depicting the character without consequences for his future role.
Momiji and Rachel are imo considered with all the effects DOA girls, otherwise we couldn't explain how TN decided to include them in the pool for DoAX3 (excluding instead Phase 4 than is an original chara of the DoA series). In addition Momiji is overpopular in Japan, while Rachel inherited an evolved moveset of an old guest character, just like a way not lose that moveset. For these reasons I can't see them excluded at all.
Sadly Alpha-152 has good chances of being cut, considering how she is not popular and lately Team NINJA seems to want keep characters with story relationships (see what said over about the DLC characters, with Raidou brough back to life and Nyotengu replacing the dead Bankotsubo) and not characters that are in the roster but aren't canon as DoA4 did.
Anyway, even if I think that the entire roster of DoA5 less the VF ones and eventually Alpha will be reconfirmed, this doesn't mean that they will be included all of them in the first version of DoA6. Very likely some old characters will get their addition later through remakes or DLC. I truly wish that this will not be the case, but sadly I don't have many hopes considering the trend of KT...
PS: I forgot about the Gen Fu role in DoA5, sadly this seemed to hint that in the next game Eliot would have kept both his fighting style and the one of his master, ending so to replace him. I hope that considering the popularity of Gen Fu at least on a competitive level (Sweet Revenge constantly remind to everyone how much the old kung fu master is deadly), TN can reconsider this matter, I really like him since DoA3.