Changes to Current Weapons
Hunting Shotgun: More pellets. This thing should be a wall of lead, but in its current form it spits out like five pellets, three of which miss at all but point-blank range.
Crossbow: Replace with a sniper rifle of some sort. In a game where manly men with loud guns slaughter chainsaw zombies to a heavy metal soundtrack, the crossbow just doesn't fit the aesthetic at all. A .50BMG bolt-action would be awesome, something that makes a very big flash and a very loud bang.
LAW: Needs more power and a larger splash. Seriously, this thing should not take three direct hits to kill a Scrake on Normal.
Suggestions for New Weapons
G3 / FAL / M14: More powerful and accurate than the Bullpup, but also more expensive, heavier, semi-automatic, and cannot carry nearly as much ammunition. Modified by the Sharpshooter perk.
USAS-12 / AA-12 / Jackhammer: Fully-automatic shotgun fed by a drum magazine. At close range, it shreds everything into fine pieces. The drawbacks include its high cost and heavy weight. Modified by the Heavy Support perk.
M60: Heavy machine gun with high power, high fire rate, and very large ammunition capacity. It's also very heavy, slows its user down while he fires, and its accuracy quickly degrades after a few rounds are fired.
MP5A4 / P90: Light submachine gun with a reflex sight and a high rate of fire. Very accurate even while moving. Low damage. Modified by the Commando perk.
Claymore: Not the tripmine, think of the Claymore from Highlander. Same damage as the Axe, but with extended range. Primary fire is a sideways slash perfect for sweeping the heads off a group of weaker enemies. Alternate fire is a mighty overhead swing, designed to deal higher single-target damage. Modified by the Berserker perk.
Napalm Launcher: Similar to the LAW, except this weapon's rockets leave a large area burning for an extended period of time. Damage-over-time from Napalm is more powerful than that of the Flamethrower. Modified by the Firebug perk.
M16 / AK47 / G36: Medium-sized assault rifle with underslung grenade launcher on alternate-fire. Rifle handles much like the Bullpup, but with a 30-round magazine. Grenade launcher has the same power as a hand grenade. Modified by the Explosives Expert perk.
C4 Charges: Remote-detonated explosives. Two of these occupies one weight slot except if the Explosives Expert perk is taken, in which case they have no weight and a maximum of twelve can be carried. Extremely powerful but tricky to use. Modified by the Explosives Expert perk.
Suggestions for Perks
Explosives Expert: As the name suggests, new perk modifying explosive weapons. LAW and Hand Grenades taken off Heavy Support and added to Explosives Expert.
Firebug: For an extra charge, the Combat and Hunting shotguns can be loaded with incendiary ammunition (lights the target on fire in addition to dealing its normal damage).
Commando: In addition to its current effect, it increases power and accuracy on all firearms to a limited degree.