The whole reason we have any of the weapons in the game that we do is because they are iconic to the genre.
Pick any weapon in the game - chances are you can find it in at least one or two other zombie ( or zombie like ) games, or games placed in a similar World of Darkness-esque setting that somehow involves the undead as a major part of the universe.
So how would adding guns or various explosives be good compared to adding a katana or <insert regional/ethinic weapon here>? It's still catering to the nature of the genre no matter what you add.
Anything outside of the genre, though, such as miniguns, cannons, high-powered sniper rifles, magic weapons / tech weapons - they are all the fan service everyone seems to be complaining about when it isn't their particular niche being addressed.
If you want to keep a certain weapon out of the game, a good reason other than attacking the group who likes it would help the argument more than the mentioned attack.
The whole problem is that we have weapons that don't follow a good progression - when they become available, the only reason not to take them is a more important purchase comes first, like armor or ammo for your secondary. Is there any particular reason to not take the hand cannon when playing sharpshooter? Or the axe / chainsaw as a berserker?
Or, to be more direct - is there any reason to go back to a previous weapon once you have purchased the next tier? Aside from getting tired of the chainsaw's idle noise, not really. Downgrading doesn't serve a purpose in the game as there isn't really a good enough reason to do so at any time as far as tactics are concerned.
Debating that becomes semantics, though, because changing from the support to commando pretty much requires selling the hunting shotgun and switching to the fairly weaker bullpup, from a numbers point of view, is a downgrade in overall power per shot. But the bullpup has a massive clip, doesn't have as much recoil, and still does decent damage as the commando. So is it still a downgrade?
I'd instead like to see any added weapons serve the purpose of covering the gaps between other weapons. I also don't want them all shoe-horned into being class weapons, except for maybe a weapon or two for the commando since he's being pretty much ignored right now aside from the recent buff.
Examples:
Another melee weapon ( katana, sledge hammer, etc ) that has high attack power and reach, but is slower than the axe. Would be priced between chainsaw and axe.
A machine pistol. Would do damage between the .50 and the 9mm, have a medium clip ( maybe 20 or 25? ) and would be dual wielded like other pistols. Firing speed would be somewhat slower than the bullpup and fairly inaccurate.
Some form of AK / AR15. Clipsize would be around 30 for balance's sake. Would do more damage than the bullpup, but not enough to make it a "only noobs use bullpup" weapon. Firespeed would also have to be comparable.
A multi-barrel riot shotgun like the one from Shadow Warrior, where you can load in up to four shells and fire them all off in quick succession. Rather than have a clip it eats away at like the combat shotgun, I'd like to see it need to be fed the shells manually with the reload, taken from your total ammo. Damage would be greater than that of the hunting shotgun to make up for a slow reload time. Would weigh as much as the LAW, slow down the player like the LAW / Flamethrower / Chainsaw, and would be priced between the LAW and Hunting Shotgun. No perk would affect this weapon.
Single use deployable explosives. Trip mines / propane tanks / land mines / larger-scale fire bombs / etc.
Alternate ammo types for all weapons and related price adjustments on said ammo. EG, support could switch from shot to slug for their shotguns and have the equivalent of a cannon, crossbows could get chemicals on the tips or shoot explosive bolts, pistols could have incendiary ammo, rifles / automatics could get rounds that cause bleeding, etc.
Stuff to add the personal choice aspect to the game rather than just buff perks is what I want. Definite trades you have to make because you personally want that over the other rather than choosing it because it's the most powerful of your choices.