Sure.
It bears some similarities to the Hackmaster system of old, but with less formality. The reality is, once you understand how D&D combat works, then consider how things like wrestling, boxing, (American) football, rugby, and similar sports work, then you don't really need much of a formal system around the order of actions. That leaves the combat system to address the resolution of the actions themselves, greatly simplifying the process of combat.
For example, you and your 3 companions turn the corner in a 10'-wide passage in a dungeon, and down the hall you see 6 orcs guarding a door at the end of the passage. Three are armed with crossbows, and all have axes. What do you do?
As soon as you start describing hostile actions I, as the DM, will tell you that one of the orcs is turning toward the door. The other 5 start approaching slowly down the hall, crossbows raised, with the remaining two holding their axes behind them. What do you do?
Assuming the party attacks, the wizard states they'll cast magic missile against the orc turning toward the door, and the rogue (who was carrying their bow in their hand) decides they will loose an arrow as well, hoping to strike the same orc. In my campaign, that orc has partial cover due to the other orcs in the way. In this case, the rogue misses, so we roll to see if it strikes one of the others. It does, so they make a second attack roll against that target instead.
The fighter draws their sword and lets out a battle cry, running down the hall. All 5 orcs let out a roar, and the three loose their crossbows, and the other two charge past them after they do.
The cleric has been holding back, and decides the fighter needs some help, so casts spiritual weapon to attack ahead of them. In the meantime, the rogue has dropped their bow, and is running down the hall as well, drawing their short sword prepared to attack whoever the fighter is attacking.
The orc that was hit by magic missiles is fumbling with getting out its keys when the wizard strikes it again with more magic missiles, dropping it. In the middle of the hall, the fighter, spiritual weapon, 5 orcs, and the rogue meet in melee. Only two of the orcs can get at the others, so the fighter and rogue attack one, and the spiritual weapon the other. The spell will strike first, followed by the others. Initially the attacks are just made more or less simultaneously (I'm rolling and so are the players). When one blow from the rogue is enough to kill one of the orcs, we make an opposed initiative check, and the rogue loses, so the orc landed its last blow against the fighter before being cut down, and the one behind it pushing in for the attack. The fighter goes down with that last blow, having been hit by one of the crossbow bolts earlier. Its ally had turned to try to get through the door to sound an alarm. It needs to get the keys, and then unlock the door after running back down the hall. That will take around 3 rounds or so, and may be too late based on how the combat is going so far.
Anyway, here you go:
COMBAT RESOLUTION
Combat is resolved roughly in real time, with the players and the DM describing what each combatant is doing. It is measured sequentially in rounds, and usually a given exchange of blows or actions is resolved together. For example, if there are four PCs fighting 6 orcs, and one of the PCs attacks an orc, that orc's action will typically be resolved at the same time.
Characters are free to react to whatever is happening during the combat and alter their actions.
COMBAT ROUNDS
Combat is measured in a sequence of rounds. A combat round isn’t an exact measurement of time, it can be thought of as about 1 to 5 seconds.
An action might resolve in one or more than one round. An attack takes one round, casting a spell usually takes more than one round.
ORDER OF RESOLUTION
For the most part, players and the DM will announce their intended actions, then proceed. Players can discuss briefly among themselves, but are encouraged to do so quickly. Actions can be altered as the round unfolds, based on actions taken by others and their results. There is no initiative or table order, let the DM know what you're doing, and the DM will ask for clarification if needed.
Actions themselves are resolved in a logical order based on the amount of time they are likely to take. In most cases, this order is not likely to be important. Most of the time, a complete exchange of blows will be adjudicated at once, unless the actions of others might interfere.
In addition, it's usually not necessary to know what action occurs first. For example, if your fighter is attacking an orc, and you'll do some damage, then it attacks back and does damage to you, it really doesn't matter who strikes first unless one of those attacks is a killing blow.
Rather than providing a number that always applies for each action, here is a suggested order of resolution, considering any movement needed:
• Completion of casting a spell (including those with a casting time of Instantaneous).
• Shooting a loaded bow or crossbow.
• A thrown weapon.
• A thrusting (piercing) weapon.
• A finesse weapon.
• A normal weapon.
• A heavy weapon.
• A reach weapon that is not being thrust.
The order might differ due to specific circumstances. If more than one action of a given type occurs in a round, and it’s important to know which resolves first, then those involved can make an opposed Initiative check:
d20 + Dexterity Modifier + Proficiency Bonus
If appropriate, Intelligence or Wisdom may be substituted for Dexterity.
CASTING TIME
The casting time of a spell is noted in the spell. A spell occurs first in the round it is completed (note that most spells take more than one round to complete in our system).
EFFECT DURATIONS
The 5th Edition rules often tie the length of a condition or effect to the start or end of your (or the target’s) turn.
• Saving throws or effects that end at the start of a creature’s turn occur before they take any actions in that turn.
• Saving throws or effects that end at the end of a creature’s turn occur at the end of the round.
If, after considering these rules, there are any questions about when an effect ends, or a saving throw takes place, assume that it impacts the target for at least one full round. A duration of a minute will usually last an entire combat.
LOADING TIME
Ranged weapons have a loading time. Once a missile has been loosed, it takes that many rounds before you can shoot another one.
MOVEMENT
Movement can be taken at any time. In general, it’s assumed that you are always in motion, circling, advancing, drawing back, and so on during the course of the battle. Unless the battlefield is large, it matters little if somebody moves 30 or 35 feet in a round.
MULTIPLE ATTACKS
If a creature has multiple attacks with the same weapon, then the subsequent attacks occur at later times in the round, often with other attacks or actions occurring between them. For example, a fighter that gets 2 attacks in a round is using a longsword and shield. He attacks with his longsword first, then his target strikes back, then the fighter takes his second attack. Opportunity attacks happen immediately, regardless of whether they just attacked with that weapon or not.
If they have multiple attacks with different weapons (claw, claw, bite, or wielding two weapons, for example), they can make the attacks simultaneously, or in a different sequence.
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Note that this is tailored for our system as a whole, which has a number of key differences.
We originally tested this with the 5e RAW, and I think I have most of what you'd need covered to drop it into your game. Movement is a big difference for us, because it's no longer tied to your turn. It also generally doesn't provoke opportunity attacks, because we're not as concerned about specific positioning (we don't use minis or a battle mat). For things like Sneak Attack, if another character is engaged with the same target as the rogue in melee, then they are "within 5 feet."
AoE spells are adjudicated pretty much the same as we did in AD&D. It starts by considering the terrain and who is engaged with who in melee. Outside of the creatures we identify as definitely being targeted, there are usually others that might be. They get advantage on their saving throw, and take half damage if they fail, and no damage if they succeed, unless it's some other non-damaging effect. As with all of my adjudication, when it's unclear I favor the PCs.
