A cool idea @Pugmaan though I don't watch enough Arcane to offer much more than that sadly ><
As it is, I recently read a
story for a setting that would make an excellent RWBY fusion,
Stand Still & Stay Silent, it has two completed arcs at 560 pages and is an incredibly well built out world, story and cast of characters.
Much like Remnant its very much a death world where people live in clustered fortified settlements but need to travel outside their walls to battle monsters, in this case the partially magic, partially genetic monstrosities that used to be people. There's a cool mix of a localized magic stem, fascinating blend of technologies and interesting back and forths across language barriers and more!
I'd definitely recommend the story, and as for fusion, I mean its not hard to either slot RWBY into this setting, maybe even in original locations or regions of the world, or into the Icelandic one of the canon, to adapt Remnant so that its a world like this over the Grimm. All in all, I think there'd be a lot of fun potential and many ways one could play with this that could be cool.