here's a few of my favorites from the various ideas I've thought up

The Stat Stealer (Medic secondary)
psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=156358981
+Copies a specific attribute from the class healed
Scout: Allows double jumping
Soldier: 15% explosive damage resistance
Pyro: Afterburn immunity
Demoman: 15% explosive damage resistance
Heavy: +50 max health
Engineer: Heals nearby buildings at a slow rate
Medic: Natural health regen rate doubled
Sniper: Needles are not affected by gravity and have no damage falloff for 5 seconds after healing
Spy: Disguises as an enemy Medic when the Spy is disguised
+25% heal rate
•UberCharge provides Medic and patient with a speed and jump height boost

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The Speedster's Scatter (Scout primary)
+33% clip size
+Gain a speed boost on kill
-20% damage penalty

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Sticky Roller (Demoman secondary)
•Stickies will roll across the ground like grenades
-20% explosion radius
-50% clip size
-4 max pipebombs out

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The Welder Smelter (Engineer melee)
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=76084965
+20% building health
+On Hit: target is engulfed in flames
-15% swing speed
-50% damage penalty
-On miss: Hit yourself. Idiot.

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The Provisioner (Heavy primary)
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=108582471
While weapon is active:
+5 health regenerated per second on wearer
+20% faster spin up time
+10% ammo regenerated every 5 seconds on wearer
-No random critical hits
-40% health from healers on wearer
-No ammo from dispensers when active

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High Octane Jarate Dispenser (Sniper secondary)
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=156077433
+Blocks one backstab
+Automatically light arrows
+On backstabbed: creates a ring of fire, igniting enemies
-On backstabbed: Ignites wearer
-25 health on wearer

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The Soviet Souvenir (Sniper primary)
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=270819448
+80% faster firing speed
+100% faster charge rate
+100% max primary ammo on wearer
-50% damage penalty
-No headshots

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The Outback Outlaw (Sniper primary)
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=394267616
+On hit: Jarate target for 2 to 8 seconds based on amount charged
-20% projectile speed
-Headshots deal mini-crit damage

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Lightning Rod (Sniper primary)
+Ignites enemies at full charge
+20% damage bonus
-Loses accuracy when fired too rapidly
-50% charge rate
-Fires tracer rounds
•Vaporizes killed opponents

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The Mutilator's Machete (Sniper melee)
+30% damage bonus
-Cannot be mini-crit boosted
-20% explosive damage vulnerability on wearer

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The Vineyard Vial (Demoman melee)
+On kill: 8 seconds of guaranteed mini-crits
-20% damage
-No random critical hits

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The Doctor's Device (Spy melee)
+Disables buildings for 3.5 seconds on hit
- Cannot see enemy health
•Vaporizes killed players

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The Hero's Hacksaw
+Damage increases with Uber level, up to 105 damage pet hit at 100% Uber
+Third successful hit in a row always crits
-20% firing speed
-No random critical hits

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The Doctor Blocker (Scout melee)
+Alt-fire: Launches a ball that prevents opponents from healing
(Opponents hit by the ball will be completely unable to regain health from anything except a resupply locker for a length of time dependent on how far the ball flew)
+100% critical hits vs overhealed players and players being healed
-20% firing speed
-No random critical hits

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The Hellraiser (Heavy primary)
+Getting kills and assists charges a Sacrifice meter. When the meter is full (8 kills or 16 assists), taunt to gain critical hits for 8 seconds
+15% fire damage resistance on wearer
-20% firing speed
-Cannot be crit boosted by outside sources
-No random critical hits

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The Pneumatic Pounder (Engineer melee)
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=5232
+40% fire rate
+Sentry upgrades directly from level 1 to level 3
-Sentry costs 400 metal to upgrade
-30% damage penalty
-33% repair penalty (thanks, Pancake Mafia!)

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The Ultimatum (Demoman primary)
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=100639814
+Enemies and buildings hit are coated in paint, marking them for death
+20% larger explosion radius
-35% damage penalty

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The Groundbreaker (Demoman secondary)
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=100639340
+25% damage vs buildings
+0.3 sec faster bomb arm time
-40% damage vs players
-4 max pipebombs out
-50% clip size

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The Supply Chain (Scout melee)
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=430963219
+On hit: Restores 20% of reserve primary and pistol ammo and recharge meters
-25% fire damage vulnerability when active
•Uses the Sandman's taunt kill; taunt killing an opponent will fully restore the clip, ammo, and recharge meter of all of Scout's weapons

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The Bloodletter (Scout secondary) (Based on Flying Guillotine)
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=431552996
+Throw at your enemies to make them bleed!
+Long distance hits cause mini-crits
+100% bleed duration
-100% recharge rate

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The Street Slugger (Pyro melee)
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=412616900
+Deals knockback on the targer
+15% damage bonus
-25% firing speed
-This weapon deploys 30% slower

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The Commando's Cut-Throat (Spy melee)
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=442881637
+Silent Killer: No attack noise from backstabs
+20% firing speed
-This weapon deploys 25% slower
-20% explosive damage vulnerability while active

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Jumpy Saw (Medic melee, name not final)
+Grants Double Jump: The second jump deals 15 damage when used
-50% damage penalty

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The Meteor Shower (Heavy primary)
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=319956399
+Knockback on the target and shooter
+20% faster spin up time
-20% damage penalty
-35% slow move speed while deployed

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The Biowaste Pump (Medic secondary)
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=151944588
•Right click uses 20% UberCharge to "irradiate" teammates, causing them to glow green, move faster, jump higher, and deal more damage at the cost of reducing their health by 33%. This effect lasts for 12 seconds
+15% UberCharge rate
-20% heal rate

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The Holo Hurtstopper (Demoman secondary)
http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=315205814
+Can store three charges at once (the meter on the HUD is split into three)
+100% charging turn control
-50% shield bash damage
-10% fire damage vulnerability on wearer