There are so many possible candidates...
• Tengu - most anticipated to return
• NG import - people who wanted Irene Lew
• VF import - people who wanted Aoi/Vanessa/Eileen/Lion/Wolf/Jean/El Blaze/Goh/etc.
• Ryo (Shenmue) - Hayashi did mentioned he wanted him in this game
• New character - likely to happen
If Team Ninja would make an inclusion thought for the fans of the DoA series, Tengu should be the main choice: the unique old character left behind from DoA5U in possession of a completely unique fighting style and moveset. This would be something not only able to satisfy a part of the fandom, but also to include more variety in the game considering it under the pov of a fighting one.
Team Ninja seem to be really fond of the old Ninja Gaiden Trilogy for NES (NG3 it's a big tribute for who played the old titles, and also NG1 and 2 have many homages to it) and in particular just of the character of Irene Lew/Sonia, that ended to get two different character models on purpose for her in games where she remained a not playable character (DoA Dimensions and NG3:RE - for not considering the subtle allusion to her in NG3 Vanilla). Like Team Ninja I really like her and I'm pretty sure that, after the inclusion of the more popular NG girls Momiji and Rachel, she will become definitely a playable character in DoA. Anyway I would prefer that Tengu could have the priority on her (for obvious reasons, being a lesser popular character that really need of the support of the fandom and overall an old character that could make DoA5U more complete in comparison with the past of the series): so, also if with my displeasure, I will not ask more for the inclusion of Irene, at least not before the return of Tengu.
In an interview Hayashi said that the Team received many requests for including Aoi or Eileen from VF, but in the end they decided for Jacky also because they thought that the game had already enough female characters. So they have a chance, but honestly I think that the VF inclusion ended with Jacky, in the end bringing to DoA5U the 4 VF characters more popular between the veterans characters (so from VF1) of their series (and the fact that they are the main characters of the anime series mean something). By the way, like I said more times, while I love (and suck to) VF and its characters/gameplay, I don't wish to get more guests in DoA5U: I prefer that the developers could give us characters fated to stay in the series (old ones, from NG or original) than characters thought for being included only in an episode. I know that this way to think is not the mind set of a game fighting player but rather of a fan of the DoA series, but I really can't help for it.
About Ryo from Shemnue, I think that the words of Hayashi got more importance than they had effectively: I think that the leader of TN expressed only his personal wish to play one day with Ryo in DoA, not that this was part of the plans of the team for the series (also because I could see something similar only after or close to the release of Shemnue 3: would be strange and "bad" to see the return of the character in a fighting game that itsn't the sequel of his main series so requested by its fans).
After the - pleasent for me- surprise of the new character Marie Rose, I can't exclude completely the hypothesis of new characters (preferably male ones). By the way Doa5 got already 3 new original characters (Rig, Mila and Marie Rose) + 1 character already known but with an original moveset (Momiji) + 1 character already known and with an half original moveset being thought like spiritual successor of Spartan-458 (Rachel). Personally I think that could be more likely to expect an old character from Doa or VF than a completely original one never seen before.
