1. Secondary Weapon(Pistol) Why not? It's kinda weird having 1 weapon, also the pistol would have a good purpose of switching between weapons in a close firefight instead of jumping around while reloading.
2. Leveling - Not I know what you're thinking. COD. Hell no. Instead of unlocking new guns, attachements etc. you should get more ammo and blocks. I find ammo to dissapear a lot, leaving you useless in firefights( another reason for a pistol).
3. Picking up ammo - This is a well known feature. If you kill an enemy which has the same type of weapon as you (SMG, Rifle , Shotgun) you can collect it's ammo.
4.Limited sprinting - I think a stamina meter and limited running would make this game more challenging, also reloading should be restricted while sprinting.
The question is - YES or NO?