My ideal game would have weapon slots like Doom 3, where you have a melee, a pistol, a shotgun, a machine gun, a rocket launcher etc. You could carry all of these guns at any time and switch them out at any time. However, you would have other weapons/ enemy weapons that act slightly differently with visuals and stats, but if you wanted to pick it up you had to switch out whatever weapon was most like it. Say you had a machine gun, and your enemy has a cooler machine gun. You would then switch machine guns, but keep your BFG 9000 for later use.
This would mean weapons had various stats to attribute to. Maybe a shotgun that used explosive damage, or a rocket launcher that launched several low powered rockets. That way you could pick up the weapons you liked to "specialize" how you killed enemies, instead of specializing in one fricken gun. I absolutely HATE the 2 weapon limit. It isn't fun, and no one in their right mind should think it is fun. How can you say that walking around firing one or two guns all the fricken time doing the same fricken thing every fricken where? (My mom told me I swear too much). Only people who take comfort in repetition enjoy that, as far as I can see.
That's why I have ALWAYS loved doom 3, quake, unreal tournament and the duke nukem games more than I ever will than Call of Duty.
Now I am not saying call of Duty isn't fun, I think the gameplay mechanics make it quite a fun thing. But there is nothing fun about firing the same gun to kill everyone the same way each and every time until you find exactly the same looking gun on the ground that fires exactly the same thing except it does it slightly faster. No, that isn't fun. It's addictive and repetitive and it adds nothing to the game.