Idea 1 – Use Surgery as an equivalent to the Pathfinder spells of lesser restoration, restoration, etc. Ability damage, drain or afflictions could be cured using surgery. This would be a pretty simple port over, and allow for some monster/attack variations in game.
Idea 2 – Use Surgery as the method of recovering max HP lost from Massive Damage. Massive Damage affects a character when they take 50% of their hit points in a single attack. No fort save. The effects of Massive Damage scale up as you take second, third or (jesus) fourth massive hits. For ease of record keeping, we’ll say Massive Damage gives the character an “Injured” condition that has multiple levels.
Injury level 1: Max HP reduced by 10%
Injury level 2: Max HP reduced by an additional 15%
Injury level 3: Max HP reduced by an additional 30%
Injury level 4: Max HP reduced by an additional 35%
This means that you can continue fighting/adventuring with level 1 injury (90% max HP), even press on with level 2 injury (75% max HP) but once you hit level 3 or 4 if injury, you really will need a surgery (or two.) This allows us to introduce the game mechanic with real in game effects, but without the “well, I guess we need to abandon this mission now” effect that we saw from save or die spells in the Pathfinder game.
Idea 3 – Use Surgery as in Idea 2, but model injury levels similarly to idea 1. Injury levels scale up with effects (possibly random?) that range from a permanent shaken condition, loss of movement, ability score damage/drain, negative levels, blindness, deafness, staggered etc.
We could also use both Ideas 1 and 2, to give Surgery a bit more depth. I think Idea 3 is ok, but I think it advantages physical attacks even more than they already are in a modern setting game.