I've been a long time Rumble player and I've always thought that Rumble's skins needed some sort of Ooomph to it. His skins seem to be fine but what I'm going with this is that there's a need for a High Technology skin similar to Pulsefire Ezreal. Mechanical stuff had always fascinated me and I've thought up a similar concept to Aetherwing Kayle for a custom champion of mine... I've even finished a concept art but alas I saw Aetherwing like a month or so after I cooked up my idea and declined putting it up as it would only look like a rip-off now. (I am still unhappy about it.)
So now, I decided to try to make a Legendary skin idea for Rumble. My inspiration for the idea would no doubt be Pulsefire Ezreal and the Battlecast series. Outside of League of Legends, I'd have to point out the Armored Core series and somehow a whole lot of mecha anime.
In the grim dark future from which Pulsefire Ezreal came from, the Pulsefires fight a war against the Battlecasts. A friend of mine actually told me about this as an idea and I've always been fascinated by it. I think Jayce is the one who invented the Pulsefire system, one reason I think that made Ezreal as part of the project. However, not everyone would like to try this experimental new thing that even Jayce himself didn't try it out (perhaps). Ezreal's one adventurous dude, so I think he'd be one to volunteer for it.
On the other end of the spectrum, the Battlecasts I believe would be part of Viktor's machine revolution. The battlecasts have no organic material left by far as visuals are concerned, Pulsefires are somewhat a mix of man and machine. Where rumble fits here would something regarding his feud with Heimerdinger.
The Pulsefire systems are developed in Piltover if I am to follow the above logic. It is nearly impossible to say that Heimerdinger has no involvement in the project and Rumble hates Heimerdinger. He's also arrogant enough to not ask for help so he'd surely not join Viktor but being a scavenger (I think) he'd still want to prove Heimerdinger wrong.
Now the Hypercore idea is to have Rumble remake Tristy from scavenged parts from fallen Battlecasts and Pulsefires. The Hypercore is a fusion of both cores, but with Rumble's mechanics it would be in an unstable state but nonetheless this suit is made from a mixture of Pulsefire and Battlecast Tech.
When below 50% heat, the core glows a pulsefire blue. When in the danger zone the core turns red similar to Battlecast cho gath's and the core sends out blue colored electricity through the tubes, hydraulics and inner workings of the suit. Once silenced, the core blinks from red to blue periodically, exhaust comes out from the vents and the chainsaws would rotate while blue electricity courses out through them.
Normal color for flame spitter is a pulsefire blue colored flame and the flame comes out from the right arm nozzle. There's a stream of fluid flowing through the right arm tube when flame spitter is active. On the danger zone it returns to the default orange-y color flame and both arms send out the stream. These are just visual effects and would not affect the range of flamespitter whatsoever.
The core grows brighter (whatever color it is depending on Heat.) The hood goes down and covers rumble, the reverse legs retract and boosters go off to move the mech.
Not much change, each harpoon is stored in the side compartment. Perhaps just have better electricity particles on hit? The size increases depending on Heat levels?
I can't really call them rockets anymore ever since the change. Rockets must deal impact damage, they always do. So based from this, I've made the Equalizer concept for this skin to have wires attached to it. The 6 tubular rocket-propelled modules shoot off towards the position you want. A stream of electricity flows through the wires and activates the Equalizer's damage per second part. After the duration the wires pull the tubes back to their position at the back of the mech.