There are so many options. He just needs to be something different than what he is now.
Off the top of my head, his passive could give him a movespeed or ad buff when he takes more than 100 damage in one second, with the damage threshold scaling up based on level.
Burning Agony could have a DoT effect, enemies who get hit by 3 ticks of it in a row will burn for an extra two ticks, to make him a more sustained threat. Or the fire's damage could increase for every five seconds Mundo has it on for.
I don't think Infected Cleaver should be %hp damage. It's very powerful when maxed out, and forces his other two damage skills to be very underwhelming. The skill itself is not supposed to do damage. It's supposed to give safe lane farming and chase down enemies. If its
damage was lowered and changed to flat scaling ad or ap, Mundo's other two offensive skills could be buffed so they can do their jobs instead of Q doing everything.
I like his ultimate, it makes you feel powerful, like you're ready to fight! But when you start fighting... the rest of his kit just sucks the enjoyment and complexity out of it. There is no strategy or skill involved in pressing w and e every time you go to hit someone.