Well, in Halo 2 you could use two guns at a time:
-Magnum
-Plasma gun
-SMG
-Needler (alien weap, shoots purple needles and then they explode)
The plasma nade sticks to the body of the enemy
In Halo 4 there are load outs, like in the CoD mod we have, but you have to lvl up your rank to get them. You can find more detailed info about them in here:
http://halo.wikia.com/wiki/Loadout_%28Halo_4%29" onclick="window.open(this.href);return false;
I mean, there are a lot of weapons in the halo universe, maybe adding them could be something cool too.
But then again, it seems like a lot hell of a scripting job doing all that...