I don't like any of these suggestions to be honest.
As anyone who plays a specific game with a LOT of weapons knows, adding more and more and more and more weapons is not the answer. In my case I used to play a HL1 mod called The Specialists and it had plenty of weapons. Instead of adding "flavor" to the game, many of the weapons felt interchangeable or were so weak they were useless, and the game was plagued by balance problems up until the bitter end.
In my opinion the loadout in Killing Floor doesn't need MORE MORE MORE interchangeable weapons, but interesting and unique is indeed welcome.
Before you suggest your favorite boner-inducing gun, THINK about what it would do in the game. Is it substantially different or fills a different niche than the other guns? If not, then why put it in? I really like subscribe to the Unreal Tournament theory of FPS weapons: different weapons fill different roles, none of them are interchangeable. Now that's an over the top scifi game that doesn't take itself to seriously, a more "realistic" game won't have that by nature, but we can still try to emulate it.
Think, why does the P90 have to go in? Wouldn't it just be a bullpup with 10 more shots?
Why must we add a minigun in every FPS ever? The idea of man-portable miniguns has been debunked a million times over. Plus, any implementation of a minigun that's anywhere close to its actual real world effectiveness would break and unbalance any video game. Hence, they get nerfed so hard that... they aren't even fun to use anymore.
Trench knuckles? Stop reading the Zombie Survival Guide or whatever.
Before you say you want to throw in your favorite gun, consider the atmosphere of the game. Killing Floor is not a glamorous, stylish shooter with loads of tacticool flavor. It's supposed to be a depressing, gritty and bleak romp through the most forbidding places in Britain against the most despicable zombie chavs. Given that, weapons that look too "new" or "futuristic" should be left out, in my opinion. The lever action and crossbow fit in perfectly, a pimped out polymer assault rifle or SMG doesn't.
Short version: Don't try to pollute the game with more boring or interchangeable guns. Consider carefully what kind of weapon would fill a role and fit into the game.