Well, I put what I think would be a fun kit in the other thread so I guess I'll slap my perfect kit here.
For the Hydra Splatling: Ink Mine / Ink Storm.
Ink Mines are Hydra's perfect sub, fullstop.
In short; It locates, it chips, it's relatively cheap and it's a deployable.
The fact that you can always be aware of what's going on in a specific part of a map based on whether or not your mines are active is huge for the weapon.
It can give the weapon an early warning when someone tries to flank it. And if the mines are set up closer, it can save Hydra a bullet or two off the kill time. Which isn't a big deal when Hydra has a fullcharge, but makes a difference when it's stuck with partials.
Mines are also very good at helping Hydra keep aggressive positions.
For example, a Hydra on the enemy stack in Barnacle and Dime with mines set up in front of it is basically impossible to get rid of without the use of specials.
You can't approach it when it has a full charge. And you can't approach it when it doesn't, because the mines will chip you down.
You might've expected to see Tenta Missiles as the special. But here's why I think that wouldn't work:
Tenta Missiles are nice in that they give location utility, they do chip damage and they paint. Problem is that they don't last.
By the time you have a partial charge ready to go after firing, your missiles have already landed with your enemies scattered to the winds.
Tenta Missiles are a nice fit for Hydra when someone else is using the special for it.
Ink Storm does all the same things as Missiles without these downsides.
Ink Storm is quick to activate, you can go back to being Hydra immeaditely. Already massive on it's own.
Storm's entire thing as a special is painting and doing chip damage. And this effect lasts much longer than Missile's all while affecting a way larger area.
Storm's location effect isn't as accurate or specific as Missile's, however it does clearly show you where there aren't enemies. This lets you focus on areas outside your Storm, which means you're potentially giving your teammates in the Storm crossfire support. Awesome!
And as an added bonus, Storm has the new healing effect! Even more supportive capability that you wouldn't get from Missiles.