i think

-no need to reload
-refils itself (1 shot for 1 sec)
-8 shots
-15% faster movement speed
-pop meter 250 dmg = next shot is a crit
-no random crits
-5% slower deploy time

id say small clip size 4 bullets in clip -10 dmg penalty its has sugar bar and it gives you a random soda effect and it get mini crits faster movement speed higher jump height and on kill the enemy turns into carbonation like bubbly air and if you hit your teammates it gives them speed and faster movement speed for 2 sec

my idea for stats
+weapon has no need to reload
+teleporter is 1 second faster
+dispenser gives +10 health per second and gains ammo/metal 10% faster
+15 sentry health and 20% faster firing speed and 10% range
+25% faster mini deploy speed and range
-50% damage penalty
-does not reveal invis spies
-sappers destroy buildings 15% faster
gun has 24 shots, can only be refilled by large ammo packs or resupply cabinet.

Stat idea.

Pros:
+Gives knockback on hit, akin to the Force Of Nature (high pressure soda)
+Enemies are slowed to 80% of their walking speed for two seconds (sticky root beer)
+Enemies are considered "wet" for two seconds after being shot (akin to milk or jarate)

Cons:
-33% damage penalty (soda obviously hurts less than bullets)
-33% clip size (four shots instead of six)
-25% slower reload speed (has to pump to pressurize the soda)

This would make The Fizzy Pop more of a support shotgun for after an Engie has set up a nest, blowing away and slowing any threats to their nests at the cost of an effective weapon if they're caught outside of their nest.

Concept: A shotgun that would be useful for sentry nests and team charges, but is not a direct upgrade from other shotguns

Proposed Stats:
(+) each shot lowers target's speed, up to 25% (stacks to five) [Sticky soda, but not stunning]
(+) +50% ammo refill from friendly dispensers [soda+dispenser= soda fountain]
(+) +10% firing speed [Maybe it's shaken too fast?]

(-) Cannot gain ammo from pick-up [Same logic as how scout or heavy can't get food items from ammo] [this is also so it doesn't overshadow the other shotguns]
(-) -30% reload speed [IDK about this one, it's a weaponized bottle of soda]
(-) -60% damage penalty [Not intended as a killing weapon, more as a support for your sentry and team-mates]

I mean it would have to be a support weapon, of course, because engi need's something like this.

Stats Could be something similar to this:
[+]Pros[+]
+ Lowers target's speed by 30% (Non-Stackable, with Range Falloff)
+ Sugar Meter Allows for shots to slow Bonked scouts and Uberchargered classes
+ 10% healing from your dispenser (Across the whole team, not just for you)
+ Crits instead of Mini-Crits

[-]Cons[-]
- 15% Damage Reduction (Including your sentry)
- 20% Damage Vulnerability
- Marked For Death when Sugar Meter has been used
- 33% reduced clip size

[]Trivia[]
= Requires 250 Damage to fill Sugar Meter
= No Random Critical Hits
= Doesn't require Ammo (Still need to pump it to reload, Kinda like the Pomson, or the Mangler.

What do you all think, anything that could be tweaked? Maybe it could have a 25% damage Reduction instead, or a 60 Damage Vulnerability against Sentries, instead of from everything? I really like the idea of a more support based primary for engi, It would really bring a fresh play style to the class. Perhaps the Crits instead of Mini-Crits could be scrapped?

+This weapon does not require ammo or reload
+Deal 350 Dmg to activate SUGAR RUSH
SUGAR RUSH:
Lasts 12 sec.During duration,gain:
+40% Fire speed
+22% Move speed
Deals mini-crits
-Cannot switch off The fizzy tapper during SUGAR RUSH
- 15% move-speed for 3 sec after SUGAR RUSH
-15% Fire rate
-10% damage

+Knockback to user and enemies hit by it
+No need to reload
Being healed by a Medic or a dispenser fills the Fizz meter. Activate with right-click to get 3 seconds of mini-crits and dramatically increased knockback.
-20% damage
-15% accuracy

+Knockback to user and enemies hit by it
+Use reseve ammo
+Players hit by get wet for 5 to 15 seconds (depending how acurate you are)
+Chance to release a shot with double the knockback
-25% damage
-less bullets per shot (only 3)
-no random crits
-while active no ammo from dispensers

shoots soda. either a continuis stream or in bursts, maybe it could switch between? idk
reloads all at once, like the pistol
when coated by soda, all damage taken are mini-crits
can be used to put out fires on teammate (+4 ammo for doing so, or +20 hp)
-50% damage
-50% damage falloff
-50% ammo

Well here's my stats :

Upsides :
+Coats Enemies in soda.
+Shots are 40% more accurate.
+50% Damage bonus.
+Reload reloads all ammo in clip.
+Soda's duration cannot be shortened by cloaking or getting healed.
General :
Enemies coated in soda count as wet.
Soda is team colored (reddish on red, bluish on blue).
Soda lasts for 7 seconds.
This shotgun reloads 60% slower, but reloads the entire clip.
Downsides :
-20% Fire rate penalty
-50% Less Ammo In Clip.
-50% Less Reserve Ammo.
-Soda overrides any other substance. (Such as mad milk and jarate.)
-Deals 0 Damage to non-soda coated enemies.
-Only shoots one bullet.
-Can only coat three enemies at max.

Unqiue MVM Upgrades :
Friendlier Soda 100$ 1 :
Point 1/2 Soda doesn't override other substances.
Point 2/2 Now deals regular damage to non-soda coated enemies.
Better Soda 300$ :
Every point increases damage to soda coated enemies by 15%
Point 1/3 Can coat up to six enemies.
Point 2/3 Can coat up to nine enemies , enemies take minicrits if the damage they take is over 75 , unless its a crit.
Point 3/3 Can coat up to twelve enemies , enemies take minicrits if the damage they take is over 50, unless its a crit.

i think it needs to be a shotgun reskin

good idea

most likely the most balanced idea and I wouldn't even mind

+rep
Sounds like a cool idea.