Going by things i've read online, there is no covenant, so adding them to the game in any way would be pointless if they're not a big part of it to begin with, the flood however, i dont know, they could be in there and if they are, it would be nice to see them in firefight. And in regards to the weapons, as far as i've read also, the cheif is basically on his own this time, so throwing in ever weapon from every other game couldn't really work either since theres again, no covenant and there's no UNSC, so unless he remembered to pack his luggage before leaving, i cant imagine finding BR's and such all over the game, but taking all this into account, its possible that the tech for these weapons came from wherever it is that the Chief went, so its still plausable.