In the end if you think about the bounties for a moment they are used as the translation of the level of threat percieved by the WG for the wanted people but on the other hand what is the meaning behind issuing astronomical bounties fir emperors, but even lesser pirates like Luffy and the supernovas, when the only people strong enough to collect them are just pirates, marines (they will not take any bounty, it is their job) or revos (even more wanted than the pirates)?
Potentually guys like the corrida fighters (Sai, Ideo etc. At least before joining Luffy) are the targets for those bounties but at the very best they can hope to take down a supernova after making allaiance and they should be the most extraordinary non pirate/marine/revo people we have seen so far. Elbaf, Wano and Zou doesn't count since they are outside the WG system.