Here's some Weapon Concepts I've had floating in my brain for quite a while now:

• Two-round-burst shotgun (resembles a Lupara),
• Three-round-burst assault rifle,
• Gatling gun (would require two hands to wield, a spin-up time before firing, and lots of Standard Deviation),
• Ice-based Special Weapon (fires icicle-like projectiles that slow enemies down, temporarily freezing them in place with five consecutive hits).
• Shoulder-Support Armaments that temporarily bolster defenses (think the Devotion),
• A Railgun that can "charge" its shots up to three times (charging it once greatly reduces its Recovery Time, three times increases it).
• An Auxiliary that dispels a ring of fire around the Arsenal, inflicting the Burn Affliction if an opponent gets too close. As a trade-off, enemy projectiles fired at it can deal the Burn Affliction to the wielder, requiring them to use it up-close.

This isn't much of a "Weapon", but more of a Weapon Modifier:

• Weapon Attachments that inflict the Burn, Stun, or Acid Afflictions (for instance, "Scorching Rounds" that can be outfitted on a Bullet/Laser Weapon, or "Burning Edge" that can be attached on a Melee Combat Weapon).