I dont exactly hope for any rating basically. It doesnt matter to me that much.
Im just thinking that the devs shouldnt aim for any rating either.
As long as there wont be a lack of blood from when you shoot a biological enemy or the lack of splatter marks. Also if there is flood then it should be as gruesome as ever with severable limbs and such.
The game doesnt need to be insanely brutal but maybe just something that has been in Halo before. What was in something like original trilogy (just some of those, there are quite a difference for those too) should be fine.
Though the transformation event of a host body with infection form could be improved a bit given last time it seemed to skip some phases. It should still be quick but just not some puff, transformed.
Also, would it hurt to have some memorable quotes, dialogue &small battlefield chatter with some being humorous, swearing, or otherwise attempts to provocate the enemies to make them doing idiotic mistakes on their anger?
There could be profanity filter option, its not like it would be the first game to have such options as turn of blood effects or change/bleep swares.
So I have no preference over what rating there should be on the cover but what I do have is preferences of what there should be in the game that effect the rating.