Tells you I've been playing DS3 too much, but...
A RWBY fic where Jaune's (see, I almost wrote "Jeanne," there) Semblance is basically the Dark Souls level up mechanic: he builds up a "charge" from killing Grimm or defeating human opponents (maybe just dealing damage instead?) and can "spend" that charge to increase his abilities in increments. Naturally, the higher one of his abilities is, the larger the charge must be before it can be spent on it. He could probably also do a Dauntless (from Worm) sort of thing and upgrade his equipment (within a certain maximum) with the charge, too.
The Deathstalker from Initiation would be the first boss, or a miniboss. Probably a boss, like Iudex Gundyr. Maybe.
"Soul of an Old Grimm," maybe, from which you can build a greatshield with great physical block and maybe high fire resist? Or "Soul of Deathstalker Serket." Then "Soul of Nevermore Pallas," which...would transpose into a sorcery staff or a sorcery in DS, but since those don't exist in RWBY...
Man, I would love to see it, but it would be a pain to write.