I've a few ideas of my own:
Splatomizer: Works kind of like what
@birdiebee described for a spray can--a bottle or tank attached to a nozzle that sprays out ink vapor. Said vapor cripples foes with a temporary "poison" effect they can resist by hiding in their own ink. However, you have to aim really close to the ground to ink turf; these weapons' loadouts would need to contain bombs and/or specials that can ink reliably to offset this handicap. I can see a few weapon manufacturers tackle this differently--either Dolphin Turf Solutions or Neo Construction would make their Splatomizer out of a perfume or lotion bottle, while Custom Power appropriates an old fire extinguisher. Splat Basics would also collaborate with Splash Mob for the variant on their own version.
Caulker: A weaponized caulking gun with caulk tube attached. Sprays a constant stream of ink as long as the fire button is held and there is ink in the tank. Ink efficiency, range and user mobility would vary by manufacturer. If Splat Basics made a Caulker, they'd collaborate with Rockenberg for the variant.
Pumppack: A backpack with a hose attached. Works like the Caulker except it needs to charge up to maximum pressure in order to have maximum range. Again, ink efficiency, range and mobility would vary by manufacturer. Inkline would be more than glad to sponsor the variant of Splat Basics's own model.
Splatboard: A skateboard with paint rollers for wheels, simply because the game didn't already feel 90s enough. You splat opponents by landing tricks upon them, and to stop riding it you have to become a squid. The Splat Basics version would have its variant sponsored by Zekko.
Bowler: Lobs a ball of ink upon the ground. The ball would then either 1) gradually shrink on same-color ink or unpainted turf (speeding up on the former), or 2) slow down and expand on enemy ink until it explodes, splatting any foes nearby. The ball will also explode upon contact with a wall or enemy. The key factors distinguishing different manufacturer's models would be initial size of ball, rate of size and speed change, and lob speed. Custom Power would make its Bowler out of the frame of a circular saw, while War Deco would use a black plastic bowl. Meanwhile, the Splat Bowler would have its variant sponsored by Takoroka.
I should probably crack open Photoshop and make mock-ups for these weapon classes.