Personally I feel to exclude Nikki, Rio, a new VF character (the collaboration with Sega seems to be "ended" considering that we got no more news under that side from long time), Shiden or Miss Monday.
Yaiba is a possibility but considering how KT acts, if they feel that NGZ got a low success in the West surely they will not release ever this character (a shame imo).
I think that Genra has to be excluded, I don't think that TN want bring back his persona. On the opposite I believe that his fighting style could get many chances to return (maybe even more now that TN opened the scene to energy beams with Raidou and partially with Nyotengu too).
If Team NINJA want go with a character already existing, the only possibilities for me remain Irene from Ninja Gaiden and Victor Donovan (that the PC release revelaed to own a full body model and face). However, while the first one could get a new fighting style, I can't see Donovan to get something completely ever seen before. In my opinion he would end to get Genra's moveset (apparently he has a bond with the Hajinmon sec) or anyway something ninja related (because you know, the antagonist of a ninja has to use a ninjutsu moveset, at least partially
). So if people are looking for something new under the gameplay, Donovan is not the right character IMO.
For me the most likely chance for this first DLC character is that she will be a female and very likely a never seen before in both the design and the fighting style. I think that if they are planning to use something old they will go with it later, while the first character could be just the best news that they can offer, similarly to what happened with Marie Rose followed from other 4 characters inheriting moveset or appearences from the past.
Just my 2 cents on what I believe could happen =)