One idea could be a bounty hunter who is initially after a character from RWBY but then discovers they are on the wrong side. For example, hunting Ruby at first because she's thought to be a dangerous criminal, but as the story unfolds, realizes she's actually a hero. Another idea is a bounty hunter who has a special weapon or power unique to the RWBY universe, like a weapon that can transform into different types of dust - powered tools. A third idea could be a bounty hunter who teams up with the RWBY gang to take down a greater threat, like a powerful and elusive criminal organization.
For an RWBY bounty hunter fanfiction, consider a story where the bounty hunter is a Faunus. This Faunus bounty hunter could be facing discrimination while trying to do their job. They might be hunting bounties to prove themselves in a world that often looks down on Faunus. Another concept is a bounty hunter who specializes in hunting down rogue Huntsmen or Huntresses. These former heroes have turned to a life of crime, and it's up to the bounty hunter to bring them to justice. And a cool idea would be a bounty hunter who has a cybernetic enhancement, which gives them an edge in combat and tracking their targets in the RWBY world filled with various challenges.
Well, you could write about a bounty hunter who has a mysterious past related to the Grimm. Maybe they were once a victim of a Grimm attack and now they hunt bounties to get revenge on those who might be involved with the Grimm in some way. Or perhaps a bounty hunter who is actually an ex - student of one of the academies in RWBY. They left for some reason and now make a living as a bounty hunter, but their past catches up with them when they encounter their old classmates. Also, a bounty hunter from another kingdom could come to Vale in search of a high - value target and gets caught up in the local RWBY - related chaos.