I'm having a geek moment here, but if they are in fact doing a Darkstalkers 4, the following key characters should be revealed as playable and addressed to progress the story:
1) Anita (In the previous games, it's been implied she'd awaken to a destiny to be ruler of humankind when she grows up, given her abilities as an esper. This is a direct contrast to Morrigan's destiny to be ruler of demonkind in the Makai upon being able to wear the Isis RIng, the Aensland clan's heirloom. It's a strong possibility that the two of them will end up becoming rivals when their full power comes to realization. In that logic, Anita should be the main character of Darkstalkers 4.)
2) A member of the Voshtal clan (In the Makai there were three ruling families-- the Aensland clan, the Dohma clan, and the Voshtal clan. Morrigan and Jedah were playable representatives of the Aensland and Dohma clan respectively, and the only reason the Voshtal clan weren't revealed so far was because they acted more as intellectuals rather than rulers by an iron fist. It would be interesting if one such representative was revealed as playable by now...)
3) Baraba Kreutz (According to official sources, this lesser noble of the Makai was not only an old rival of Demitri, but also somehow linked to the werewolf Gallon. He is technically not confirmed dead, but had "disappeared". He would still be an interesting character to help expand on Gallon's story upon accepting his curse as a werewolf.)
4) Emperor Ozomu (The fat demon responsible for taking over the Dohma clan by killing Jedah 100 years prior, and giving the zombie Zabel his powers in the afterlife. After Jedah was revived, he was killed by him. It would still be interesting if he was somehow revived by a member of the Voshtal clan only to be used by that clan to rule the Makai.)
5) There was also a demon blacksmith in the Makai who created Bishamon's armor and sword mentioned in official sources that should also be taken to consideration. Now that Bishamon has become a "master" of Hannya, it's likely he'll want the armor back, as the fact a human has control over the armor may prove to be a bad omen for the Makai. This would help expand on Bishamon's story a great deal.
6) Not necessarily essential, but I heard there was also a rumored new character named "Will" (according to early development rumors), who is apparently another Darkhunter like Bulleta. Based loosely on William Tell, he wields two shotguns, but mainly uses them melee-style.