I kinda like the way you think it should look. Maybe smooth it out a little and make the barrel longer to preserve the MA5C style of assault rifle. My vision of the assault rifle in Halo 4 looks like what is shown with the tweaks I mentioned.
The magazine size can stay at 32 rounds and have a kill time of around 16-20 rounds.
The rate of fire could be slightly faster than the Halo 3 assault rifle's rate of fire, but only slightly or it would possibly be OP.
The sound could be a mix between the way it sounded in Halo 3: ODST and Reach so you could still have that Halo 3 feel with a slightly more rugged sound.
And finally, the reload sequence. I would like to see a reload sequence similar to the Halo: CE reload sequence: pulling the charging handle back, removing the empty magazine, slap in a fresh one, and bump the charging handle back forward. If I had the software to recreate my vision of the reload sequence on the computer screen, I would show you. Oh, and if you're reloading mid-magazine, you simply remove the mag and slap in a full one.
Please, tell me what you guys think. I don't mean to brag, but I believe this would be an excellent assault rifle and would fit perfectly in Halo 4.