Rumble main here. I'm convinced his winrate is only low because he suffers from the "Noobs play him" problem. He is hard to pick up but once you get him down he's easy to play well.
Assuming he DID need a buff, the LAST thing you'd want to do is buff his ultimate. At worst, they sidestep it as it falls out of the sky. At best, their whole team is baited into the jungle where they all take 60% of their hp from full duration. Buffing the Equalizer would make him and incredibly toxic one and a few halfs trick pony.
If he did need buffs, they would need to buff his w. His q is great, and with its good scaling and high bases, is fantastic danger zone late game. His e is again, bad early unless you land both, but once you get it leveled and some ap, can be once of the most powerful abilities and the only reason he can duel and adc late. His W is never really great, it's just always decent. And to buff it, they would have to remove the shiled OR the speed, and move that power to whichever they leave behind. Or they could keep both and increase the cd and make it stronger.
Qwe makes him great in lane, and his r keeps him from falling off in teamfights. He has more viable build options than any other AP champions aside from Amumu and Cho'Gath.