I did this a while ago! It was big on Twitter for a bit. I forget the exact thought process for everything here so some holes might be there in how I got to an answer but trust me, I did think about this lmao.
At the absolute minimum, I wanted every class to have TWO weapons. I chose the default weapons of most classes but do want to explain to where I got the second of each. The absolute baseline was as follows -
Splattershot, Splat Roller, Splat Charger, Heavy Splatling, Slosher, Splat Dualie, Splat Brella, Blaster, Inkbrush, Tri-Stringer, Splatana Stamper - These are the defaults of each class and I saw no reason to remove any of them.
Splattershot Jr. - Needs to be here. I could see a case for it to be gone if people find bomb spam too annoying but bomb spam can happen regardless of whether there's a Jr. on a team. I like what it does for introducing players to the game and it has a pretty distinct identity for a shooter.
Dynamo Roller - I wasn't super involved when this weapon was a top tier but it's an AoE weapon in a class that doesn't normally have them. Not many weapons have this excessively slow of startup before they can deal any form of damage, even the chip of usual slower weapons, and I like that you have to play around that. Sick weapon.
Squiffer - One of the most unique midlines in the game. You could argue that this is similar to the REEF-LUX but I don't think they're quite the same.
Ballpoint Splatling - Liiiittle bit of bias here but there weren't too many backlines here and I like weapons that are this flexible, especially in a context like this. Do you want to play a shorter-ranged Splatling? Well, you might not get exactly that...but maybe this will be close enough.
Explosher - Similarly, there's not many AoE backlines...period. There's also not many backlines with good painting. I'm going to start going over these a bit quicker because I'm now realizing just how much I'll need to go through lmao
Tetra Dualies - A fan favorite and the single most mobile weapon in the game. Leans the furthest into a dualie's gimmick and it's awesome for that.
Tenta Brella - One of the hardest weapons in the game and it lends itself to a completely unique playstle nothing else has.
Rapid Blaster - This is the stand-in for both Rapid Blasters. Kinda cheating, but you can't have one without the other. The fact that it doesn't one-shot is sick for a blaster and the slight flexibility is cool.
Octobrush - Maybe this is the default of the class? I dunno, I just think Dynamo is better than Painbrush. This class has not been done much justice honestly.
REEF-LUX - There aren't many weapons that can paint as well as a short-ranged Shooter. Plus it's sick.
Splatana Wiper - There needs to be an easy Splatana and there aren't many hyper aggressive Quick Respawn weapons out there at this point.
Then I got rid of some weapons by default...
Aerospray. N-ZAP, Splash-o-Matic, Carbon Roller, E-Liter, Hydra Splatling, Sloshing Machine - Not the same, but similar enough playstyles to the base weapons to the point where it's easy to drop this one. The exception is the Aerospray which is a worse, more unhealthy and bad version of Splattershot Jr.
Clash Blaster, Painbrush, Undercover Brella, Bloblobber - Unviable weapons that are tough to balance and a nightmare for newer players as is. Cool ideas in a lot of cases but not worth it.
Squeezer - Accessibility problems would need fixed...again, not worth it.
.52 Gal - Everyone hates this weapon and there are multiple short-ranged shooters already as is. Goodbye.
Splattershot Nova, Jet Squelcher - There are better Shooter options for similar range values and both of these have devolved to paint spam at one point or another.
L-3 + H-3 Nozzlenose - Unique weapons and I love L-3, but they kind of muddy up the identity of the Shooter class. They're not easy to use and are not super flexible.
Big Swig Roller - Paint spam weapon. Just play a Shooter or something.
Goo Tuber, Bamboozler - You could argue that the Squiffer does a similar enough job to these two.
Tri-Slosher - Again, people don't really seem to like this thing. In hindsight I totally could've added this because a short-ranged weapon that can fight and not paint is pretty cool.
Range Blaster - I'm not gonna act like this is the same as the Rapid Blasters but you could add more different Blasters than this.
So this is what I'm left with.
First off, sure, let's add all three of the Shooters. I like what this weapon class aims to do and there needs to be a lot of them for them to work. Pro adds a straightforward midline, 96 adds an inaccurate but powerful weapon which is very unique for this game, and Sploosh plays to a Shooter's mobility.
I wanted to add a midline Splatling as well and this was really a toss-up between Nautilus and Hedit. I love Mini a lot but it can play much more closely to a Shooter than the other two. I chose Hedit because Nautilus is a bit too easy for how strong of a main it is and it maintains Hydra's big distinguishing factor as well. This really is a toss-up though, could go either way.
I also wanted a midline Dualie from here. The choice was super easy. The Dualie Squelchers are a really unique, sick, and deep weapon, and everyone loves them even in spite of how weak they are. I know Dousers and Gloogas have plenty of fans and I personally love all three but the choice was pretty clear.
Also...Decavitator. Come on. I have to keep it. Do I need to pull up the essay I wrote on why I love this thing? It's impossible to not be biased here and let me just tell you there's a reason I felt so much more strongly towards this weapon than any other in the game.
Flingza doesn't play to its range outside of being a worse Roller with painting when it's not sharking. I'll pass. Snipewriter can stay for another painting backline that also kinda sorta but not quite fills the Bamboozler role again. Dread Wringer feels almost like a middle ground between a regular Slosher and Explo even though, again, not the same, and we already have Tetras for the super super mobile Dualies, so I axed both. Luna could be covered by adding S-BLAST and that would also kinda sorta cover for Range Blaster missing.
I did all this a while ago but for the sake of this, let's just say the last two were Wellstring and Recycled Brella. One needed kept, one needed axed. Everything else felt super black and white to me thus far for keeping vs removing, and this was no different. Wellstring is a weapon you could argue outright outclasses the Tri-Stringer, is still similar to the Explosher, and many people find it annoying to play against, while the Recycled Brella is a super sick idea that, while it's a ways from being viable right now, is worth keeping by the merit of having a completely unique playstyle. Obvious choice.
And that's it! Was pretty fun doing this.