If we look at the CS weapons, it's pretty obvious at least three (and a half) fall out of the running right away:
Polar Star. It's a weaksauce version of Spur, with no upshot.
Rocket Launcher/Super Rocket Launcher. GG2 already tried this, and "weaker rocketman" is well covered by INSmod.
Machinegun. Everyone else does machineguns, and QC already has booster for movement.
If we look at the others:
Fireball. Yes, it has a bounce mechanic and a limited lifespan, which would make it unique. It's also straightforward, there's no need to worry about its different levels as with the Bubbler or the Sword as the Fireball gets straight, uncomplicated upgrades.
Bubbler. It's three weapons: a semi-auto, accurate, but very short-range and weak gun; an inaccurate and limited range but moderately powerful rapidfire weapon; and that strange shield/charged spread/machinegun. Which are you going to use? Obviously the first choice is worthless, and the second choice is moderately generic, but is the third really going to fill any kind of role any other weapon can't, either?
Sword. The first and second level, while similar, have somewhat interesting differences: the first level has moderate range, but benefits most at point blank; the second level doesn't reach out to the same range, but at point blank is enormously more powerful. And the third level throws the other two out and just does its own wide-area dispersion, which is pretty powerful too, if low in rate (discards the established closer/more damage mechanic). Since INSmod enjoys characters with high power levels, would it be okay to use the third level? If not, is it worth using the other versions when GG2 has spoiled this idea a little already?
Snake. The last time INSmod had a wallpassing weapon, it was revoked. Granted, this is shorter range, but it's also rapidfire, sustaining damage. It would play to a purely defensive stance, I feel, and you'll have to check against spawn zones to reduce spawncamping. (Like the fireball, I don't remember any of the levels being significantly different besides in power.)
Spur. This is a chargeable beam launcher. It's probably a really safe choice, and moderately unique, especially since Samus only charges one loop max, and this charges three loops. If you're including this, is it worth also including the Whimsical Star bonus?
Nemesis. How do you get the lower level/stronger damage mechanic to work? Do you even bother? (Yes, I know you already said this, I'm just going over everything I can think of.) While it's terribly powerful, it doesn't repeat on hold in CS. Is that worth keeping here? If your strap-on partner is using it already, is it worth having it yourself?
And that's only from a purely CS perspective. I mean, sure, not everyone is satisfied with how GG2 handled it, but would straight CS versions of QC have worked? Would they work in INSmod?
Personally if it were up to me their attack would be throwing mutant rabbit people and that would be the end of it