Ok, so i played rumble recently, and, dear god, is his model old. His mechanics are pretty outdated to. So i'm making this discussion to just point that out. For starters his overheat passive is far from polished and the way he is built it feels like overheating is far from what you want and just poking from behind is preferable. His q orients itself to the way your walking, which on one hand means you can move freely while using, and on the other hand, it feels weightless, making it more like ornn's fire breath wherein he orients the flames in one direction and gains a shield would be better. His w- it's nice giving him a shield but it isn't large enough to make enough impact on his tankiness and the move speed could at least be a dash to get him into combat in a more fluid way while also letting him hop walls. His e- electric harpoons is actually what i like most, a mid range poke and slow for lane harass, yes please, it could just use the usual polish for old abilities. Onto his ultimate. Now I believe riots intent was in part to let him use this as a way to close off pathways and annihalate enemies in tight spaces, but almost every champion has a dash of some kind now or the tankiness to push through it. The damage is actually quite good and the range is very nice, it can help you assist in any teamfight especially those over baron or dragon, just drop it at the mouth of the pit and engage, enemies take some burn damage and with liandry's it racks up. However it feels more like the ability of a poke mage, not a harass ap bruiser, it's range is so long it might as well belong on corki considering how it looks. So pick a theme, update the abilities around that.
Thematically rumble(according to his lore) is a disgruntled yordle inventor who enjoyed cobbling together scrap to create gadgets and often gets in fights thanks to his quick temper. In the end of his short story Rumble shows up in piltover to declare that he will show all those gosh darn hoomans what real yordle tech can do. So his kit and gameplay should reflect this with Rumble as he pushes his gadgets to their limits to defeat his enemies and show that yordle tech is the best
Here are a few ideas I like
Passive- Overheat fills up a heat guage on your screen, the higher the heat the more damage your abilities deal but the longer the cooldown- reaching full heat causes ability cooldowns to not tick down until you exit combat- this means poking let's you deal moderate damage but if you want to deal the most you have to be careful and pick the right time to fill up all the way or else you could be stuck without abilities for a while- this fits the theme as rumble pushes his gadgets to the breaking point to show up everyone else, dealing increased damage even if it means breaking your abilities for a while
2nd part- Scrap metal- taking a certain amount of max health damage will cause rumble's mech to spit out scrap as it breaks- rumble can pick up these scrap's and when he is not in combat rumble will add the scrap onto the mech as armor
-this fit's rumble thematically as he works to repair his mech so that he can keep fighting on and display his ingenuity and tech