I think, Halo in escense cracked the inventory system for weapons.
However, it was too harsh as a third weapon would have been handy.
If you consider that you can have a gun holster for your pistol, pouches for your grenades and the rifle can have a sling leaving your hands free for a third gun.
What I'm saying is, have a weight limit providing you can find slings, rucksacks, extra pouches and gun holsters.
You should then be able to carry more weapons and equipment obviously with an obvious hinderance to your mobility and endurance.
Another nice touch would be that you can see all weapons and equipment on you and the NPCs as seen in Operation: Flashpoint and Counter-Strike: Source to name a few games.