It seems that Wizards is already experimenting with the idea of token cards, you can probably just follow their lead. Their cards explore the concept of token cards existing in your library, graveyard, hand, and your opponent's library and hand.
I'm probably going to keep these spells to the hand and stack (maybe battlefield) for now (As much as I like Bone Rattler). I originally wanted to make primarily token instants and sorceries, as they are the only ones who don't have tokens right now (and I think it would be neat to generate spells in hand), but if I get a neat idea for a token permanent spell, I might change my mind on what card types token spells can be.
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https://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=476173
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