I suggest making weapon upgrades more significant by giving them unique behavior which makes the upgrade more desirable, important, and/or satisfying, and give Flamethrower and Rocket Launcher an upgraded version.
Here are my own ideas for the upgrades, but I'd be fine with whatever upgrades you guys come up with - I just want some kind of significance added to the upgrades.
- Pistol / SMG: The pistol is now semi-auto and fires once for each key press. The smg is unchanged and remains full-auto.
- Double-barreled Shotgun / Combat Shotgun: The double-barrel is semi-auto like the pistol and can fire two shots at the current rate of fire, but then has a long pause before it can fire twice again. The combat shotgun is unchanged.
- Flamethrower / Napalm Launcher: Flamethrower would work like a tank's flamethrower that does less damage (spitting a short cone of fire in the direction you aim but not lingering on the ground) and Napalm Launcher would be identical to the current flamethrower.
- Rocket Launcher / Bazooka: They are identical aside from a simple rate of fire increase. Maybe it can upgrade damage, range, and/or projectile speed, but technically those should only be affected by the ammo used, not the gun.
- Bazooka (Rename the rocket launcher cause it's just plain cooler) / RPG: Currently a rocket starts out slow, then accelerates through the air and homes in on your target. Remove the homing effect (maybe give it to a new, expensive kind of rocket ammo) and make it shoot directly where you aim, like the flamethrower. Then, make 'rockets' fired from the RPG instantly accelerate to max speed, making it much easier to hit moving targets with the RPG. Also, give the rocket launcher a short minimum range, like artillery, since a rocket traveling too slow will not hit with enough force to detonate.
(This would be kind of unrealistic since in real life these are very different weapons and don't use the same ammo, but typically neither do pistols and submachine guns, so I don't think it should matter)