One could tweak how quickly Resolve is restored. If you slowed it down to one or two points per day, PCs would have to spend time resting and recuperating.Those are interesting in limiting the short-term, but again I'm more about long-term attrition. With those, you might have trouble with a running fight because you won't be easily able to heal up to full, but a day of downtime and you'll be just like new. I like the idea of having to spend a week or two recovering. I feel like 5E encouraged my players go just break-neck around the place, even with "gritty" healing on.
There’s some thing I accidentally left out when I suggested System Strain before (because WWN is way too verbose for its own good*, and I only just realized it recently): WWN requires eight hours of uninterrupted rest to benefit from natural healing (regain HD in hit points, lose 1 System Strain). That makes attrition very real while exploring because you’ll have to double the rest period in order to take watches or risk being interrupted while resting. Depending on what the party needs to do while making and breaking camp, they might only realistically have a few hours of adventuring/exploration. This is meant to encourage you to be careful but also to consider hiring people to guard your camp while you rest (making adventuring more of an expedition).In this case, I like having multiple options to heal. But I also want some limiting factor so that you don't just recover in a single day. Looking over the mechanic, Strain seems like a fantastic way of managing that: typically speaking a PF2 party isn't for lack of healing (especially with Fonts for Clerics), but I want them to have to manage that in some way. Having so much means that long-term healing isn't really something that happens... unless you put in a limit. Strain... that looks like it could do it.
Well, in theory. We’ve only had one session since that revelation, and the full impact of it hasn’t been felt yet. It will be interesting to see how my players adapt to the rest requirements (especially with the new exploration procedure we’ll be trying this weekend, since the one in WWN and B/X is kind of crappy).
The way WWN handles this is you gain the Mortally Wounded quality at 0 hit points. If you’re not stabilized in six rounds, you die. There are two ways to stabilize someone: by making a Dex/Heal or Int/Heal skill check or magical healing (magic or otherwise). If someone stabilizes you, you recover in ten minutes at 1 hp and the Frail quality (but no gain in System Strain). Magical healing works as normal (increases your hit points and causing you to gain System Strain). In Pathfinder parlance, this would be like using Administer First Aid instead of some other healing to stabilize someone, and the former doesn’t increase System Strain while the latter would as normal.The only situation I'd say that you might be able to heal from without Strain is from 0 hitpoints (which is fine, since the Wounded condition means you're not going to get much out of that edge case). In fact, you could probably do something about having levels of Wounded limiting how much strain you can recover until the condition is gone. Lot of potential in adding in that limit, just like Healing Surges were meant to do in 4E.
Frail is both nastier than the Wounded condition in Pathfinder 2e and not. If you have it, you die immediately when you reach 0 hp. You also can’t benefit from natural healing while you are Frail. To get rid of it, you need a week of bed rest, treatment by a physician, or magical healing. Magical healing removes the condition, so Frail is a non-issue for a party with a Healer.
Unlike Paizo, Kevin Crawford has stated outright that parties are expected to be at full hit points before an encounter. This is interesting because WWN is not designed around encounter balance, but that’s the expectation. In SWN, you have easy access to healing items and psychics. Because WWN is trying to be swords and sorcery (rather than heroic fantasy), magic items aren’t pervasively available. Instead, it makes healing accessible via first aid after battle (roll 1d6+Heal as many times as the healer wants, each time adding System Strain) and magical healing (via mage partial classes).
If I were adapting this to Pathfinder 2e, I’d look at the Stamina variant first because it already does most of what System Strain does while being designed around the Pathfinder 2e framework. I’d start with the default recovery of Resolve, but I would be open to tweaking it if it didn’t feel right based on how much I wanted rest and recovery to be a thing.
If I did want to adapt System Strain, I’d do it because I wanted to use it where WWN also uses it (like for privation). If you don’t have enough food, water, warmth, or shelter, you gain System Strain (particularly if exposed multiple days in a row). It makes planning and logistics much more important for exploration, but I’m running an exploration-based game, so I want those things to be important. Regardless, if I wanted to use System Strain, I’d strongly consider letting the party make multiple Treat Wounds checks as part of the same activity instead of having it require multiple activities.
Even better would be changing the activity to dispense with the check and base it on your Medicine skill some other way. Otherwise, Assurance (Medicine) and Continual Recovery (and maybe Ward Medic) end up being feat taxes, which isn’t in the spirit of PF2.
* WWN may be a bit too Gygaxian for its own good. Rules are buried in paragraphs of text and are sometimes not very well specified. There are a few gaps when it comes to exploration (and other things outside of the core adventure loop), so I ended up hacking OSE into WWN to fill in those gaps.