Technically since they are doing their own 'origins' stories, they could add anyone they wanted. If they wanted to make a chapter about Kin and Gin attacking Cloud Ninja, they could. If they wanted to do Madara vs Hashirama, they could. If they wanted to play out the fight were the second Mizukage and the second Tsuchikage killed each other, they could.
Technically there are no limits, they could put the entire fourth ninja war roster in if they wanted, but I wouldn't hold my breath. They've said they aren't focusing much on story this time, so I doubt we'll get a lot of depth or original material out of it. I'll be happy if we get the battle at Naruto Bridge (Team 7 vs Haku/Zabuza), Fights against the sound 4 from the Sasuke Retreival arc, Sasuke fighting Danzou, and Killer Bee and Guy fighting Kisame. --- PSN Herugrim666 Level 11, check out my collection! "Ham! I love ham!"