I'd highly suggest looking at RPG systems specifically designed before World of Warcraft.
Regardless of how you feel about WoW, there's no denying that the impact it had created ripple effects that stretched far and wide throughout RPG's and gaming.
MMO's tried to emulate its design in a desperate effort to stay competitive (which arguably had the opposite effect) and even singleplayer games occasionally adopted a few inspired ideas. Pretty sure even D&D 4E's changes were inspired by certain WoW-related trends.
So if you want a wider palette of ideas, you might be better off looking back just a few years. Here's a few examples I could name that have mechanics that might diverge from what you're normally familiar with:
Star Wars Galaxies
(Pre-NGE, before they tried overhauling the gameplay to emulate WoW)
Spiderweb Software
(Specifically their slightly older games, at least in my opinion.)
Mortal Online 2
(Heavily inspired by SWG, with a dash of Bethesda "do thing to improve skill in thing" approach, and a rather unique skillbook related system)
And also look into some tabletop/pen and paper RPG's that aren't based on D&D or D20. A few that I've come across (though have minimal experience with):
Burning Wheel
The Riddle of Steel
Mythras
Also going to split this off in a separate comment, since my comment was already getting a little text-heavy:
Another thought that comes to mind, loosely based on what I was reading about Burning Wheel, is how 7 Days to Die's internal dismemberment system actually works. I'd have to read through the game's XML's again to refresh my memory, but the TLDR of it was that each limb had a different minimum threshold that needed to be matched or exceeded in order to achieve a dismemberment. (Player-side modifiers include the weapon/item class, weapon quality, weapon upgrades, and relevant player skill levels).
I once considered adopting some kind of a core damage formula loosely based on that, but was on the fence given how in many respects, it's still a purely numbers game only with slightly different rules. Plus you end up going down a rabbit hole of trying to overcomplicate other things like bleed-out and trauma mechanics...
I'd highly suggest looking at RPG systems specifically designed before World of Warcraft.
Regardless of how you feel about WoW, there's no denying that the impact it had created ripple effects that stretched far and wide throughout RPG's and gaming. MMO's tried to emulate its design in a desperate effort to stay competitive (which arguably had the opposite effect) and even singleplayer games occasionally adopted a few inspired ideas. Pretty sure even D&D 4E's changes were inspired by certain WoW-related trends.
So if you want a wider palette of ideas, you might be better off looking back just a few years. Here's a few examples I could name that have mechanics that might diverge from what you're normally familiar with:
Star Wars Galaxies (Pre-NGE, before they tried overhauling the gameplay to emulate WoW)
Spiderweb Software (Specifically their slightly older games, at least in my opinion.)
Mortal Online 2 (Heavily inspired by SWG, with a dash of Bethesda "do thing to improve skill in thing" approach, and a rather unique skillbook related system)
And also look into some tabletop/pen and paper RPG's that aren't based on D&D or D20. A few that I've come across (though have minimal experience with): Burning Wheel The Riddle of Steel Mythras
Also going to split this off in a separate comment, since my comment was already getting a little text-heavy:
Another thought that comes to mind, loosely based on what I was reading about Burning Wheel, is how 7 Days to Die's internal dismemberment system actually works. I'd have to read through the game's XML's again to refresh my memory, but the TLDR of it was that each limb had a different minimum threshold that needed to be matched or exceeded in order to achieve a dismemberment. (Player-side modifiers include the weapon/item class, weapon quality, weapon upgrades, and relevant player skill levels).
I once considered adopting some kind of a core damage formula loosely based on that, but was on the fence given how in many respects, it's still a purely numbers game only with slightly different rules. Plus you end up going down a rabbit hole of trying to overcomplicate other things like bleed-out and trauma mechanics...