In Ergo, one token per transaction can be issued, and the token id must be the same as id of the box of the first input.
Thus if you have generated a box you know id of the future token. And this id is calculated via hashing.
Thus via iterating over a register (e.g. R4) used as a nonce, you can create a token with specific id properties, e.g. starting with certain number of zeroes. So some work could be required in order to make such a token. Something like VanityGen-adress in Bitcoin, but now VanityGen token )
Then some contracts may accept such PoW-backed NFTs only.
I haven’t thought about use-cases though. Maybe some related to filtering out spam.