Fight harder, fight smarter [D20]

Combat is usually a sequence of hits or misses, to defeat an opponent, in any way possible. 

Whatever changes to this is just another way to say "hit or miss", or "how much damage you deal". 
It's not the "what gives me the best ratio between the chances to hit and damage dealt" that's interesting, but the "it's better to fly and shoot with my shortbow, or stay on the ground and hit two times with my dagger and getting hit back?" that matters, and the choice between the two when you have a limited amount of moves.

Fighting should have the same weight and dynamicity that has adventuring, it shouldn't interrupt the flow of narration but must require the same level of planning of long term adventuring.
If sometimes the fight gets boring and requires the GM to change the stakes, to make "something" unpredicted happen, why not give the same opportunity to everyone?

Here's my take on how to make fight different, deadlier, and a bit more tactic (and since the last i played is d&d 5, I'm gonna use that as a base, but you can adapt it to any system with little tweaking).

• Initiative = Highest base score modifier

Initiative score is not determined anymore from dexterity, but from whatever is the highest base score modifier. Any other bonuses/maluses (proficiency for example) applies as usual. Same for monsters and NPC. Don't roll initiative.

Imagine a sorcerer getting ready to launch her spell while she's distracting the opponents with her body language, or a fighter charging in with the raw strenght of his leg, or a druid who doesn't need to take a breath and be ready for the fight before its opponents, or a monk who waits for that moment in which sun shines behind him before making a sprint.

• Stamina = Hit Dices (HDs)

Every PC has HDs up to their total level.

You can't have more HDs than your PC level (Max HDs).

Your HDs size are determined by your class.
Acting costs HDs.

If your HDs get to 0, you can consume your max HDs instead (it's called Straining).
If you have 0/0 HDs, you drop unconscious, wheter you still have HPs or not.
You return conscious after a long rest.

Fighting is all about resistance, not just giving and exchanging hits.
Planning the moves and exhausting the enemy sometimes is more important than making him go to 0 HP.
Sometimes fights can go on for days, break after break, chase after chase, before declaring the winner. 

• Recovering stamina

You naturally recover 1 HD at the start of each turn, and all HDs up to your max HDs, at the end of the fight.
Magical healing recovers the same number of HDs you roll to recover HPs, up to your max HDs.

Your max HDs return to normal after a long rest.
Max HDs don't recover with magical healing, it requires resting.

When you have a short rest and spend HDs to recover HP, your HD type decreases in size (d12 > d10 > d8 > d6 > d4). When you expend HDs to recover HPs, now you roll max. HD + your cons. modifier.
You don't automatically recover HPs anymore during a long rest, instead your HD type increases one size, up to your class dice type (d4 > d6 > d8 > d10 > d12). 
When your dice type becomes smaller than a d4, your short rest becomes automatically a long rest.

Very good resting conditions may let you recover max HDs and HD size after a short rest (For example a royal resting place, a very able healer, a good balm...). 

Very bad resting conditions, prevents the recovery (For example sleeping in the open in very bad weather conditions, being cured by someone who doesn't know what is doing, or being ill...).

• Step initiative

Identify the factions fighting, and their objectives.
Group together whoever is fighting who has the same initiative score.
The round starts with the group who has the highest initiative score.
The faction with the member who has more HDs is the leader faction and decides who goes first from the members in the group.
Once the member has been decided, it chooses what to do and pays in HDs.
Then another faction chooses which member is going to act and does the same.
This goes on until everyone in the group has acted once.
If more than one have the same HDs, compare max HDs, if equal then compare HDs size, if equal then roll a D20, and keep rolling until someone gets an higher score than the other.
Once everyone in the group has acted, include the 2nd group to the turn, and start again from the beginning. 
Keep going like this until everyone has acted, then everyone recovers 1 HD, and proceed to round 2.

Surprise rounds works as usual.
The surprising party acts as the first group and leader faction, during the first round.
Surprising the enemy, or having advantage on initiative, gives 1 more HD to spend during the first round.

Being surprised by the enemy, or having disvantage on initiative, give 1 less HD to spend during the first round.

To help i suggest giving each player some sort of token each time their group is included in the turn 
The best way would be using playing cards. Give 1 card to the players for each turn they are going to play (so if there's gonna be 3 turn, the player playing from the 1st turn gonna receive 3 cards, the player in the 2nd turn 2, the player in the 3rd turn 1). Then start with who has more cards. Each turn they act, remove 1 card. Start again with who has the most cards, then remove 1 card from each of them. Go on until everyone has no more cards.

• New action rules 

You can only use as many HDs per turn as is your initiative score.
Attacking costs 1 HD.
If a class ability grants multiple attacks, they are considered as 1, and need only 1 roll to hit.

Moving costs 1 HD for each base speed increment for that type of movement. 

Moving changes, now each time someone is in the reach area of an enemy, his speed drops to 1 square, and goes back to normal when he's out of reach. 

Actions, abilities or spells that require an action, costs 1 HD.
Spells and abilities like these, need a turn to be spent preparing, and are launched the next turn.
Actions, abilities or spells that require a full round, cost 2 HDs.
Spells and abilities like these, need a round to be spent preparing, and are launched the next round.
Actions, abilities or spells that require a bonus action or less than action don't cost anything, don't need preparing time, but must be used together with another action that has a cost (their HD cost is included in that action).

Attacks of opportunity and reactions cost 1 HD.
Reactions can be...
Dodge: lets the opponent move 1 square in whatever direction (ducking costs as such).
Deflect: with a shield lets the opponent get minimum damage from the hit.
Counter with a weapon: lets the opponent deal minimum damage back (a ranged attack may be countered only with a ranged attack, a melee attack only with a melee attack).
Usually non-humanoids can dodge, deflect, and counter as they see fit.

When rolling to hit you can't miss anymore, instead something else happens. 
When rolling a critical 1 you do minimum damage, lose 1 more HD, and if that would not allow you to spend anymore HDs this round, your turn is interrupted. 
When rolling a critical 20 you don't spend HDs for this action, and it doesn't count to the initiative limit.
Rolls to hit above enemy CA causes the opponent to receive average damages, with no consequences.
Rolls to hit below enemy CA causes the opponent to receive average damages, and let the opponent react.

Advantage on a roll means you inflict maximum damages. 

Disadvantage on a roll means you inflict minimum damages.

Average damage is the average damage you can roll + the appropriate modifier (1d4 is 2, 2d6 is 6, etc). 

Minimum damage is the minimum damage you can roll + the appropriate modifier (1d4 is 1, 2d6 is 2, etc).

Maximum damage is the maximum damage you can roll + the appropriate modifier (1d4 is 4, 2d6 is 12, etc). 

Feats, bonuses, and maluses to damage apply as usual. 

Feats and abilities that let re-roll damages instead adds another damage die.

To help speed things up, since damage is fixed, you can write it as [min/ave/max], so [1d6+3] becomes [4/6/9].

• Optional: Excalation die [13th age]

Use a D6 starting from 1.
At the end of each round, the excalation die goes up 1.
This is added to the cost in HDs for each action.

The more the fight goes on, the more draining and difficult it becomes.

• Optional: Grittier stamina

You don't recover HDs at the start of each new turn.
You recover all HDs per short rest, and 1 max HDs as well as 1 HD size per long rest.

Fighting is excruciating, most of the times it's better to run away than to fight until one is too exausted to go on. I didn't want to use exhaustion rules because i think it's better to leave that to out-of-combat situations, or to special abilities.


So, what's the meaning of all this?
If you made your homework, it should be clear how is gonna work: PCs are going be able to make only 1 action per round at 1st lv, so move, attack, or defend, perfect to get the grasp of the system and define tasks, instead of everyone doing the same thing over and over. Then, as their level goes up and the enemies get stronger, the fights are gonna become more complex, with more actions and reactions avaiable in the same turn, and a better management required in and out of combat (the idea is very similiar to the progressive increasing complexity of Shadow Of The Demon Lord). At the same time combat will require more critical thinking and get harder for both parts, so even an enemy 10 times stronger than the PCs might risk of losing for exhaustion, and this should not scare the GM, but help them understand what combat really means, not an empty come and go of hit points, but a game of wits.

Combat is going to be played a lot like a reverse poker round, with people getting in the fight at each round, instead of getting out, and the turn ending when everyone has played their hand.
Of course not everything listed here may work, you may have to tweak some parts, maybe take only some ideas and discard the rest, or thinking that is a complete mess not worth even playing, but i would like to hear your suggestions.