Mundo's not on the rework list for a while, right? If so, please give his passive this change.
Mundo's passive is super-boring, super-invisible-power, and no gameplay. It just makes health-stacking more potent on him by making his regen super-high.
Since Mundo's not due for a rework anytime soon, though, how's about changing his passive to something more... tangible? I.E., the following:
Mundo regenerates 0.X% of his missing health every second.
Rather than having it work off his max HP and be constant, make it so he has to spend his health or take damage to get said regen. It's by no means a permanent solution, but it'd at least do something helpful in the meantime by making him less "balance inflexible".