I don't think these ideas will work, broadly speaking, because they fall under the same trap that hinders other "gritty" suggestion: They are solely and exclusively negatives for the players.
If the question is, "Would you like to stand here and do what we've always done, or would you like me to hit you in the head with a rock?", people generally don't favor the rock. But if you give them a wooden stick and a ball to swing at, with the warning that the ball might hit them, and they need to be careful not to hit themselves with the stick, they'll be quite happy.
What you need is a new beneficial mechanic that has, as a side effect, increase risks within combat. EG:
- You get a new super move if you're down to 'bloodied'! But if you miss, you open yourself to an automatic counterattack from the enemy.
- You get greater freedom of movement in combat, but so do the monsters, which means they have an easier time attacking downed players.
- Ranged cures are always AOE. That means you can heal your allies that are standing next to each other, but it also means that if you heal someone who just got taken down, you also healed the monster that took him down.
- Increased combat flexibility for players (something very much desired) means increased combat flexibility for monsters. How about a cleave attack for your fighter? That makes it much easier to hit downed players while still fighting effectively. A 'cover' mechanic can also be used to separate you from your downed ally. The ground slam your barbarian wants isn't locked behind the magic-user paywall, so AOE damage ahoy!
One critical factor is turn order. If no enemy can act between the time the party member falls and the person who can heal him can act, then it's kinda pointless. Legendary actions are great for pounding on death saves. Is there a way to make combat move more like that even without a legendary monster enemy?
And what about death saves without needing to drop to 0 HP? (A poison that can be applied to a bloodied character?) Or at least, unconsciousness, which is nearly as good.
I had an idea for a sap weapon that does 'virtual' damage. That is, the physical damage is still the standard 1d4 of a club, but it would do 2x-3x that damage to determine whether it knocked a person unconscious.
For example, Str 14 thug hits your level 1 fighter (12 HP) on the back of the head with a sap. He rolls a 3, for 5 total damage. You take 5 real damage, but 15 'knockout' damage. Since 15 damage is enough to drop you to 0 HP, you're out, even though you're then lying unconscious with 7 HP left.
Now what happens when your cleric comes along and heals you? Well, you're still unconscious, until someone slaps you awake. The healing doesn't make you "not dead", and thus able to resume fighting.
Now take that analogy to a full fight. The dragon has a bodyslam attack that's designed to knock you out. So now you're unconscious (incapacitated), but aren't at 0 HP. The cleric can heal you, but you're
still unconscious. No ranged yo-yos. You're not dying, sure, but you're absorbing more of the party's actions in order to get you back into the fight than a simple Healing Word. And that imbalance can swing things sharply against the party, especially for an AOE type of attack. (Though you'll probably be woken up by the next wing buffet.)
0 HP is sometimes described as unconscious, sometimes dead, sometimes just down and unable to fight. I'm not trying to figure out what 0 HP actually means; I just want to make it possible to explicitly go to the unconscious state without going to 0 HP.
This of course is a beneficial mechanic for parties that don't want to go murderhobo on everything. So, again, something that is useful to everyone that can also be used to make things much deadlier in the right circumstances.
Addendum: I'd like to have actions other than "Attack" be strongly useful in combat. Part of the problem, I think, is that combat is already so short that big bad monsters don't get to shine. In order to counter this, people are turning towards ideas to make things "deadlier" for the players, which is difficult to do in such a short time span, and comes across as petty.
I'd prefer an approach where combat is "more difficult", and that a boss fight (or even non-boss fight) feels like a boss fight. I don't think that's feasible when combat is so one-dimensional and short. Short, deadly fights seem to require a certain amount of cheese, and players cheese right back. We need more room in combat to
do more.