Tahm Kench has what healing needs to become.

Gray health.

All healing is reduced by 30-40%. But heals double on a champions "gray health". Or damage they recently received. Costs of heals reduced by ~25%.

This weakens passive healing. But keeps it's combat effectiveness till useful up to a point.

Now you can just remove all Grievous Wound effects. Maybe keeping it for Ignite.

Can add new defensive or utility talents that lower the decay speed of gray health as well. As well as items.