I talked about a weapon swap system in a video of mine, check it out
here. I think that a weapon swap system should be part of the game. However, it wouldn't make sense to have weapons swapped in then middle of a game, it just wouldn't work.
I get where you are coming from, and I liked your idea for team composition and voice chat option, even being able to just see what your allies are picking before you hop in can do so much to improve the game
About it not working in match, I think it is alot easier than you would expect, cuz it is already in the game, Salmon run lets player jump back to a spawn and force the player to automatically swap weapons on reset....to implement this mechanic into matches they might have to tweak it a little (like allowing the player to choose instead of RNG) it is easily possible
It might be a little confusing and take some time to adjust to, but that is why I feel it would work so well in a game like Splatoon, they start us off with a bare bones frame of a game, giving us time to understand mechanics and slowly adding weapon, gear, map, and even game mode options to help us hone our skills and play style...slowly but surely adapting to the new changes
We saw this with the new tech of the roller, charger, and dulies, then as they add the brellas and some gimmick weapons that make use of these changes (ballpoint explosher, tenta, UC brella etc.) these are just seen as useful and fun additions to the game
So in short I do believe that it is both possible and easy to implement it is more or less a question of would nintendo be able to do it and whether they would desire to do it in game