Now this may seem completely and utterly insane. Like, pretty much unheard of in terms of Naruto and RWBY crossovers. Turn the Grimm into a sentient species with a summons contract, with sub-clans with ancient versions of what types there are as the sub-bosses. Now the utterly insane part comes into play: Salem is the boss summons. Now those who had watched RWBY have come to hate Salem with passion, as she basically killed of Pyrrha and was basically a complete and utter bitch as she basically caused the events of RWBY. But this version of Salem is kind, caring, and more or less neutral with the shinobi world. Now here's some possibilities of how Naruto, or an OC I don't care, gains the Grimm summons contract:

Option 1: He finds it in the forbidden scroll of sealing when Mizuki tricked naruto into stealing said scroll.

Now for this it would be a bit tricky, as while the name 'Grimm Summons' sounds epic, I don't think it would just immediately draw naruto's attention, so if a name was there, like his mother's, Kushina Uzumaki, was there, then he would get interested, especially if you make the name 'so and so uzumaki'. however this would put Hiruzen in a bad spot with naruto if you choose to go with naruto instead of an OC, since he never even mentioned a uzumaki clan to naruto. this also puts the council in a bad spot since they either never bothered to have the shinobi academy go over even a brief history of the uzumaki, or just completely erased it. There are quite a few possibilites with this option of naruto or the OC getting the Grimm Summons contract, but it seems a bit excessive to be honest.

Option 2: Naruto is drawn to an ancient temple on the shores of what used to be Uzushiogakure no sato (Village hidden by whirling tides or hidden eddy village), the ancestral home of his clan.

Now this would be a bit easier since it would during Team 7's mission to Nami no Kuni (Land of waves), and most likely the temple would be an uzumaki temple. Said temple would have certain seals which would draw anyone with uzumaki blood within a certain radius, and would also tie into the uzumaki clan as the main summoners of the Grimm, and yet would still put Hiruzen and the council in a bad spot, especially if the Grimm hear of how one of the remaining uzumaki was in a village Uzu had an alliance with, and yet learned nothing of his clan or his clan's ancestral home. This seems like a good route, and it would be a bonus if naruto or the OC used the grimm summons during the battle on the bridge and saved either zabuza or haku, or possibly even both. (I have a soft spot for the two, so what?)

Option 3: Kyuubi knows of it, decides it doesn't want a weak container and informs naruto where it is.

Now this would be hard as fuck but you would be awesomely amazing if you did this, as early naruto/kyuubi dynamics can honestly be hard to portray without it ending up with the two ending up chummy or being complete dicks to each other, and it is incredibly hard to find that just right area with said dynamics between the two. Now for this, you can come with some random-ass place you made up within the depths of your mind, or a place already in the narutoverse. Hell, you could have naruto leave a kage bunshin with 99% of his chakra in konoha and have the original sneak out and head all the way to tetsu no kuni (land of iron) to get the summons scroll for the grimm.

Bonuses:

FIRST AND FOREMOST: CORRECT GRAMMAR AND SENTENCING! That is basically what makes or breaks a story besides what the story is actually about!

Naruto/OC gains Grimm characteristics (Doesn't matter how they get them).

Naruto/OC are neglected/beaten to an inch of their life but remains strong and friendly because of their friends if they have any (Sucker for those kinds of stories).

Naruto/OC has an individual pairing (Doesn't matter who as long as it seems reasonable).

Salem ends up being like a mother to Naruto/OC when the two actually meet face-to-face.

Put in some constructive bashing, like on sakura to make her a better person, or sasuke to get him out of his 'homicidal avenger' shell, and others as well.

Danzo is somehow killed (extra bonus if it's by naruto/OC using the grimm summons. I really fucking hate danzo.)

Pin the uchiha massacre on danzo alone (like have him forge Hiruzen's signature for the orders and stuff, make hiruzen completely oblivious until the massacre actually happens).

Well, that's it for the challenge rules/options! Hope you have fun trying to make a fanfic like this, 'cause if you do, TELL ME AND I SHALL READ.