They added both to Halo 3, and it was great. So why not add them to Halo: Reach for Custom Games and Firefight? Halo: Reach could even have it’s own version of Recon Slayer with Automag starts whenever that gametype comes up in the rotation for Halo 3.
I would even go a step further and add the Battle Rifle to Halo: Reach to be part of Custom Games, Firefight, and Recon Slayer. There are a dozen mods on PC that add the Battle Rifle to Halo: Reach, it would be awesome if those of us playing on Xbox could get something in an official update for MCC.
I believe the biggest reason they did so with 3 was thanks to the fact that they were able to port them over from ODST since both games ran on mostly the same engine, and used largely the same models requiring only a minor rework to their stats.
closest I think they can do to that now would be porting the H:O DMR and/or ODST Gauss Turret to Halo 3, and some of the Halo 4 content to H2A.
Mod authors have been able to recreate the Battle Rifle, Automag, and Brute Plasma Rifle in Reach without official mod tools, so it wouldn’t be that hard for 343 to do the same. They’ve also shown it’s possible to port weapon models between Halo 3 and Reach.