If there's one thing the much loved sewer levels need is some consequence for going down there and kill giant rodents and talking, swirling, giant, moving goos the color of poison. This notion should apply both to injuries but also diseases.
In terms of healing, this would mean that some conditions may not be curable at all without external help from a specialist. Not just in terms of healing, but in general, I'd like to see the game step away from the system that makes the player's party more or less a fully independent and self sufficient nation. Think Jag2 for this, though if it's the player character down, I suppose it'd be just some timeskip forward. Severely injured party members would need to undergo some meaningful time off for recuperation at a hospital, and you may opt to switch them out while they do so. One thing I'm less sure of, although interesting on the surface of it, is to encourage some party shuffling because of an enforced layoff due to said medical aid.
DRACONIAN POTION OF MIND FULL
But back on topic, it means I want to see fairly minor healing being able to be done in combat, some limited healing - enough to carry on but not back to tip-top shape - outside of it, and needing help to get back to full health after serious injury. Resting in proper accommodation would be meaningful, and scavenging materials should be a very limited exercise. More broadly, it means I'm also against having a comprehensive crafting and repair system, instead limiting player crafting to makeshift gear at best.