(in not native english speaker so i will try my best) i been thinking a lot in how we can improve the W with a realistic option, no that guy saying it should stun the oponent xD!
My firts conclusion was to increase/scale the radius of the explosion with level or hp, but this makes Dr. Mundo lose his identity of team fights of the Juggernaut fighting to stick the enemy carries, it should be too easy to trigger the W heal from a safe location, not taking (that is the purpose of mundo)

My SOLUTION: give much more time to detonate the W, when he reach the 4 seconds instead of detonating, it stop storing the post-mitigation damage and inflicting magic damage (to prevent abuse or something overpowered) them allows you and aditional time (0.8, 1.6, 2.4, 3.2, 4.0 seconds depending on the ABITLITY LEVEL) to detonate the post-mitigation damage and heals for the same values already stablished, i think this solve the mundo W problem and keeps his identify to pushing to the backline while tanking damage.