I mean, the answer there is to not give injuries that only affect one type of character. So, something like...
- Broken arm. Cannot be used for any purpose, including holding a weapon, shield, or spellcasting focus, or pulling out material components. You automatically fail any ability check or saving throw that would require the use of both arms, and take a -5 penalty to any saving throw to catch onto a ledge when pushed off.
- Broken leg. Your normal speed is set to 10 feet, unless something else would cause it to be lower, you must use all of your speed to stand up from prone, and you automatically fail any ability check or saving throw related to balance or forced movement.
- Skull fracture. You have a -3 penalty to your attack rolls, saving throws, saving throw DCs, and AC, and when you roll damage, roll twice and take the lower result.
- Gut wound. While you are in combat with an untreated gut wound, you must make a Constitution saving throw each time you take an Action, Bonus Action, Reaction, or begin to move (only the first time you choose to move during your turn). The DC of this save is 10 plus twice the number of times you have made this save during the current round. If you fail this save, you take 2d6 damage, or half as much on a successful save. Other than a successful save, this damage cannot be reduced in any way, not even by resistance or immunity.
- Punctured lung. You cannot speak and your speed is halved. Further, you must choose whether you move, take an Action, or take a Bonus Action--you can only do one of these things until the punctured lung is healed.
- Severe concussion. You are permanently blind and deaf until the injury is healed, and you have disadvantage on all attack rolls and ability checks. If your spells cause any targets to make saving throws, those targets have advantage on such saves.
Naturally, some folks will take some injuries more easily than others. A Con 24 Barbarian 20 who took Tough (and thus has a +13 Con save and ~325 HP) is going to shrug off Gut Wound--but one would expect that
a 20th level Barbarian could take a wound to the gut and hardly even notice. For a more typical character, though, that's
rough--spellcasters can lose concentration, and the checks
keep coming. You're on a death clock unless you get that injury healed. Broken arm is kinda-sorta tolerable for Dueling characters that (for some reason) aren't using a shield...but still sucks. That's probably the only one that needs beefing up though. Maybe force a saving throw at the start of each turn or be incapacitated (can't take actions nor reactions) from the pain? I'm sure there's something that can be done.
Of course,
personally I think all of this is...well, it just sounds like exactly the opposite of a fun time. "Here's all the ways the game WILL torture you, HAVE FUN!!!!" I just...I really don't get the point of having all sorts of horrible debilitating injuries that make risk-of-death so much more damaging, because death is a breath away in 5e even when you're getting on in levels. But if you're going to design a subsystem, it should be worthy of existing. For this, that means making penalties where ALL of them are bad options for nearly everyone and you're just trying to not suffer too badly before it can get healed.