An interesting thing I think they're pretty likely to utilize is the Japanese belief that certain more powerful spirits like Kami or Oni can actually shape-shift and transform into different animals or natural forces. Either that, or he'll summon different nature spirits to inhabit his suit, sort of similar to Den-O.
And, yeah, I think the rubber stamp thing makes a lot more sense working in a Rider belt than actual stamps. Especially if they're old & he has to hunt them down or something. Although... on second thought it seems pretty unlikely that they won't be plastic, or look anything like the image I have in my head...
Beyond that, there's a pretty varied folklore for the supernatural the world over. They could really take just about any route you could think of. Or, even, purposely utilize different cultures' folklore for different spirit enemies in different episodes.
