Carrying on with the nuts and bolts of weaponry, I think there's so much more that can be done with old skool machinary. Forget plasma cannons and rifles (zzzzzzz), it's all been done before. Let's have mechanical weapons, some powered by steam or some kind of mystical kinetic energy. So just some ideas....
Cross Bow - Wooden, bolted together with steel. Deadly, low ammo and reload rates. Headshot for a kill. Fairly fast projectile, but still need to lead the shot.
Buzz Saw - Rusty, steam powered. Takes a second to spin up, ejecting smoke and causing the beholder to counter-act the centrifugal force. Absolutly deadly in close combat - but a complete handful to use. Can be swung wildly and unpredictably with right-mouse-button
Smoke Grenade - Single power up. Restricts visibility to within a few feet. Can completely turn an area into a close combat zone for 60 seconds. Reduces effectiveness of long range weapons
Grenade Launcher - Pretty standard, but needs to be included. Grenades should be quite powerful and bounce meaningfully. Limited to launching no more than 5 at a time before reloading to prevent pineapple spamming
Poison Arrow - Single Power up. Delivered from a single hollowed out bamboo - Ok, so this is a bit like Rune Quake, which I happened to like. Limited power up. One shot - poisons the enemy for 30-60 seconds....gives hallucinations, flash backs....disorientation etc etc.
Nailgun - Plenty of ammo - Lead your shot, minimum damage and recoil.
Spear of Justice - A single use spear, chipped blade, stained with blood. Can impale enemies with a single throw. Can be retrieved by anyone who can pick it up quick enough. One per level.
Supernail Gun - Has to make a return. Massive rusty iron cannon with brass detailing. Limited ammo availability. Deadly when hit. Big recoil, but big damage. Nails remain stuck in players and objects when hit for a limited time and can occasionally ricochet unpredictably.
Thunderbolt - Q3 effectivley rendered the lightning gun a poor, impotent cousin of the Thunderbolt and essentially replaced it with the Railgun - which I think is too powerful and has no weaknesses given its infinite range. Thunderbold Needs to be reinstated Q1 style - Low cell availability, extremely powerful. Coupled with Quad - you better run. Has limited range.
Shield - Power up - Single use, instead of replenishing health or special suits. Howabout a range of sheilds, only, ranging from wooden upto solid iron or bronze...Roman/viking style defence. Sheild reduces damage by xx% and has it's own life span. Watch as it slowly disintegrates while being gunned down. Dropped sheilds from fallen foe can be picked up and used. Front facing protection only, can be attacked from behind.
Mace - This could be the fall back weapon when all else fails. Covered in blood, able to hack sheilds to pieces, very short range. Given to all players.
Blunderbuss - A ridiculously large musket, imperial in appearance with brass and ivory fixings. Has a tendency to misfire. . Super wide spread, short range but deadly in close quarters. Sometimes fails to go off altogether leaving the beholder in a cloud of smoke.
Liquid Gun - Liquid Nitogen fillet pellets are flung, slowing your opponent down gradually freezing him. You can now safely unleach your fury and shatter them to pieces with carefully placed shotgun blast. Pellets can also neutralise grenades and maybe other weapons
Headcage - Power up. A medieval defence. Stops headshots completely and reduces overall damage by 30%. Obscures view substantially. Stays with player until killed. Can be picked up from fallen enemies.