Mundo could use a more interesting passive. Here's a concept for one.

So basically the concept is that Mundo has a resource bar for Adrenaline (0 - 100) and he builds up Adrenaline by hurting himself with his abilities in combat. When his Adrenaline reaches 100, it is consumed over a few seconds and buffs Mundo during that time (sustain + Movement Speed). There are tons of effects that could be explored here instead of just healing Mundo. For example, Adrenaline helps you ignore pain. Maybe Mundo's Health Bar could be hidden during Adrenaline Rush? Or maybe damage he takes could be delayed like Death's Dance?

This gives Mundo a window of power to work towards in lane. His ult should help make that window more reliable and easy-to-access in the mid to late game. In exchange Mundo loses a little Health Regen ooc and his ult no longer gives him bonus Movement Speed, that being shifted to Adrenaline Rush. It could give him other stuff though.

His abilities could get bonus interactions to fuel his Adrenaline (like Masochism dealing damage to Mundo on-hit). His ult 'Sadism' could allow him to generate Adrenaline by hurting others with his abilities.