I played D&D a lot from AD&D thru 3rd Edition, but have only been playing 5e for about a year.  Overall, I like this version of the game, but I know some older players are a little put off by the seeming lack of risk to characters due to the proliferation of hit points and healing options.  

While I like heroic characters, I have been thinking about how it might be possible to add an element of risk to combat without bogging down the system.  What I came up with is the ‘injured’ condition.  Whenever a character takes a critical, they receive the ‘injured’ condition, which even if they heal up the hit points lost due to the attack, represents the lasting effect of injuries.  Basically, as long as you have the ‘injured’ condition, you have ‘exhaustion’ level 1 which can’t be removed until you first remove the ‘injured’ condition.  If a character is already suffering from the ‘injured’ condition and takes another critical hit, the character now goes to the ‘seriously injured’ condition.  At this point, the character also suffers from ‘exhaustion’ level 3, which again can’t be removed until the ‘seriously injured’ condition is first removed.  If a character suffering from the ‘seriously injured’ condition takes another critical hit, the receive the ‘critically injured’ condition, which also imposes ‘exhaustion’ level 5 and ‘incapacitated’ conditions, which can’t be removed until the ‘critically injured’ condition is removed.  Of course, if you think critical hits occur too often, you could allow the player to roll a death save to avoid the ‘injured’ condition.  Obviously there is a lot of nuance you could add to it, but I just wanted to keep it fairly simple.  This way, characters can be heroic but still face risks from getting into combat.

The only thing I haven’t really figured out is how do you remove the condition.  I know the big complaint will be that if it takes too long to remove, it will mess up game night if a character gets ‘injured’ or worse right at the beginning. Or that it will mess up the flow of the game.  But while I want heroic characters, I want them to consider the risks.  One thing that could be added is the idea of a character getting a heroic action if they suffer this condition, which would only be usable in a sacrificial kind of situation in a narrative way.  For example, a ‘seriously injured’ character takes the heroic action to guard a bridge against a horde of trolls, while the rest of the party is able to escape.  In this scenario, you don’t fight through the combat at the bridge, the dm just says the character sacrifices themself to hold off the trolls, killing several even as they hacked at the character.  Even though the character finally succumbed to their wounds, the party was able to escape.  Maybe that’s too much.  Anyway, curious to see what people here think.

Sounds like you want critical hits to inflict exhaustion. The rest of the system just feels like fluff. A problem with it is that it makes low-level play even more luck based. The early levels are already infamous for "crit=dead," and this would amplify this problem quite a bit. It also puts martials at a big disadvantage, as they're going to be the ones taking most of the critical hits. If you think that there are too many ways to restore hit points, adding enemies like vampires that reduce HP maximum or enemies that can kill without damage like mind flayers to your games could make damage feel more permanent. There are also optional rules in the DMG, such as insanity and 7-day long rests, that can make players panic a lot more when things go bad.

Thanks for your perspective.  When thinking about creating any kind of home brew rule, there are always risks.  While I can see what you are saying about my idea is really just having levels of exhaustion and I can see that might almost be better. Then, instead of dead, you become incapacitated at level six.  Provides a bit more scalability as well.

As to your point about lower level characters, they are more affected by every element of the game and I don’t think my particular thought on having some kind of injured condition would affect them disproportionately to higher level characters.  In fact, if you look at it mathematically, higher level characters would be more affected by this rule.  Of course, having enemies that can impact characters in other ways besides hit points is fine, but vampires and mind flashers don’t make good adversaries for low level characters anyway.  Also, your point about combat characters being more affected is valid, but criticals can occur from many damage types and as a dm, I’m going to make sure I have enemies that use ranged weapons to attack those pesky characters who like to skulk in the back.

Ultimately, if you take away the extra damage from crits and replace it with my proposal for injury conditions, it seems like a pretty fair trade-off.  I would even go so far as to say that cries causing an injury condition, possibly even specific to the type of damage taken, feels like it adds flavor to the game.

The thing is, you wouldn't be creating a new condition. You're making a homebrew rule that causes exhaustion when a monster deals a crit. There would be no reason to make a whole new condition that acts exactly the same as a house rule.

You could also use the One D&D exhaustion rules. If I remember right its -1 on d20 rolls per level of exhaustion, up to -10. Not quite so punishing. Healing would be just like normal exhaustion recovery, long rest, greater restoration. Maybe you could come up with some other recovery mechanic like restoring one level when all your hit dice are spent, or extra recovery resting in an inn rather than in the wild.

Also, to try to even out the martial/caster divide maybe rolling a 1 on a saving throw would also add an injury?