Spike should add "Sasaki Kojiro" as DLC. For those of you that don't know who he is. He was a long time rival of Miyamoto Musashi and is considered the most challenging opponent Musashi ever faced. I think that would be epic. He "Sasaki Kojiro" also holds the title of "kenshi" "master swordsman" like Musashi.
They already have data for the samurai from the first game they could just use that data to create "Sasaki Kojiro". Plus Sasaki favorite weapon was a "nodachi" and they(Spike) already have data for that weapon and fight style. It would be very easy for spike to turn the samurai from the first game into "Sasaki Kojiro". Also they already have data on a ninja. They could very easily use that data to create "Hattori Hanzo".
Besides I feel a samurai and ninja (besides a gladiator) really are the only warriors missing from this game. Then maybe for the next game they could add "Musashi" to do battle with his old rival "Sasaki" and to see how he fares against "Hanzo".
Another cool idea is Spike could make a "Hattori Hanzo" warrior from the data they already have for the ninja/samurai. "Hanzo" was a samurai and a ninja master in real life. So it would just fit perfect.