What do I want combat in D&D to be? I want it to flow conversationally, like Dungeon World. I want it to be fast, intense, chaotic, first-person, and visceral, like the Middle Earth game Silmenume plays. And I want it to have all the mechanical and tactical richness provided by modern D&D.
People talk about action economy in D&D a lot. It basically comes down to, each team has so many opportunities to make progress towards defeating the other team. It makes a fight feel like a team sport, rather than a personal struggle for survival; and it makes people feel that not spending their currency to contribute to the overall effort as efficiently as possible == sub-optimal & ineffective.
Here's a rough attempt.
Instead of rounds and initiative, have the spotlight move with the conversation. Say what's happening, turn to someone, and ask what they do. Keep going with that thread until you get a natural break point; often one side or the other's gone down, or taken some meaningful damage—or maybe someone else at the table is jumping up and down waiting to tell you what they're doing now!
When you can, move the spotlight to whoever's been most quiet or acted least recently.
Sometimes you'll spotlight an individual, sometimes a small group of characters. Occasionally the whole party. You might try to spotlight one person, but it'll end up naturally that someone else, or a slightly larger group, is the focus of action.
GM: Around the corner come three orcs and behind them, in chains, a cave troll. They're as surprised to see you as you are to see them! A, you're in the front, what do you do?
A: Rush to meet them in melee!
B: Me too! Nobody's getting past us!
When a player says what their character does, the GM says how enemies respond in turn. Often one PC to one enemy, but not necessarily! If you're surrounded by foes, when you do something, maybe all or several of them respond. Or vice versa. However many foes react, it should be enemies who are directly engaging with the spotlighted PCs, or maybe using the conspicuous respite granted by the PCs' attention elsewhere to do the something important.
In general, however much "action" the PC uses, that's the amount of "action" the opponent responds with. In general, move for move, attack routine / spell / etc for attack routine / spell / etc... with bonus actions really being bonus!
GM: As ya'll charge down the corridor, the troll and two of the orcs scramble to meet you!
Exchanges and Bouts
Exchanges trade melee attack routine for melee attack routine, among everyone in a melee group! You can give up your attacks to disengage or dodge, or use some spell or special ability that basically amounts to a melee attack, but nothing else.
In a bout of melee, the GM will usually call for 2-3 exchanges in a row! If one side wants to withdraw and the other doesn't, they're gonna have to sacrifice their attack routine in a bout to Disengage, unless they can do it as a bonus action or whatever.
By a melee group, I mean a scrum where at most one of the sides has multiple combatants. So 1v1, or a PC and the foes that surround him, or a group of PCs surrounding their victim.
Let's call attack rolls maneuver rolls instead, here. You can choose to apply your damage from each maneuver as you roll it, or you can lump everything from a whole exchange or bout into one hit, at the end of the exchange or bout, whatever you like. Errol Flynn style fence-fence-fence-fence-STAB! Or bloody, gory hack, hack, hack.
Between exchanges, players should do stuff! Like try to wrestle, outmaneuver, whatever. This is where to try to grapple or disarm or push or shove or trip your enemy, or force them over to the bridge and fling them over the edge. If you try something and your opponent wins, they get to do something of equal-ish nastiness to you, instead. If nobody quickly jumps in with something they want between exchanges, the GM will just ask for a flat d20 roll. Natural 1s and 20s between exchanges are chances for the GM to apply the chaos of battle. People trip. Mounts rear and throw their riders. Blades shatter. Shields splinter. Quivers spill. Morale gives way.
At the end of a bout, combatants push away from each other, stumble back, have a bit of a space to breathe.
GM: A, throw an exchange against the troll, AC 15! (dice clatter). He's holding 18 damage over you.
A: Fuck, nothing. I want to scramble on his back where he can't reach me!
GM: Your Acrobatics against his Athletics! 19!
A: 14! Fuck!
GM: He plucks you off, throws you to the ground, and pins you there with his foot! You're prone and restrained! Roll another bout, at disad! (more dice clatter)
A: Miss and a hit! Spending a superiority die for Pushing Attack. 19 damage and I throw him off me!
GM: He lands another claw. You take 29 total from the bout.
When the GM calls for a volley, PCs and their enemies in the spotlight sling spells and arrows at one another. Usually an exchange of ranged attack routines, with the option to give up your attacks to dodge or run for cover or cast a spell or whatever.
If you're smart and lucky, you might be able to get a volley unopposed: All the enemies are busy fighting your friends, nobody's trying to feather you in turn. If you're so free, and the action's been off you for a while, the GM might grant multiple sets of your ranged attack routine.
We'll call attack rolls aim rolls instead, here. If you like, each aim roll can be an individual arrow. Or you can bundle it all in to a single, deadly shot.
At the end of an exchange of melee, if there are unopposed enemies, they might get free volleys if nobody's trying to deal with them. Uh-oh!
What's my Speed good for?
When lines of battle are forming, faster characters or monsters get to choose who engages with whom. They get to shape the initial melee groups. Maybe you can Dash in order to choose your foe, forfeiting your first exchange in the bout of melee.
The Action Economy is Grist for the Mill
Of deciding what's "fair" reciprocity. But it's not the iron-clad law. Some characters may end up with more actions total, but likely because they're fighting for their lives, not mowing down all the opposition.
This is getting long. And I want to see what all these mysterious comments to the as yet nonexistant post are.