i understand that we do not really know of his powers but we don't no much of 2nd hokage or Hashirama yet they were both included and obito was included as well.
IDK, maybe he want introduce until the War Arc. Much of Generations is probably set before the War Arc, it's just about where the 5 Kage Conference is or after, maybe.
They could include him with the excuse of the vs Harashirama fight. I guees they will not include him to have a major character to introduce for the next game
dlc pack was never intruduced in any naruto game i had ( half of them ) and story hasnt got that far, it would get to to the pount were kages figt him then the game would just end.... not very good plotting