Can you imagine if they take SHinobi in that direction? After him defeating Zeed, ends up fighting various tropes like dinosaurs, cowboys, pirates or something like that per game.
Or make them separated.
Shinobi in Wild West World
Shinobi in Atlantic World
Shinobi in Greek Mythologies World
Sound great, right?
I always like bosses in Shinobi games :
They always look cool and scary.
Many bosses from different tropes would make it more epic.