I have not playtested this, it's just something I've been thinking about. Feel free to critique, etc.
I have mostly been running Dungeon Crawl Classics so any modification replaces the standard DCC core rules.
Standard DCC: roll initiative at the beginning of combat and keep the initiative order until combat is over. Declare your action, then roll to see if it beats a target number/difficulty check, if a hit occurs the player rolls damage based on their weapon.
The idea: each round of combat takes one D20 to cover initiative (which changes each round), attack/defense/other, and damage (if attacking).
If the PCs or Monsters have multiple attacks, they roll all of their attack dice at once, then decide how to use their roll to resolve their actions.
Players and GM roll first, then decide what actions they will take.
Initiative:
Lowest rolls go first (high rolls are more likely to end the fights so they resolve last).
Action:
Players choose to Attack or Defend (or perform some other action such as casting a spell, etc).
-Attack:
--add your usual attack bonuses (strength modifier for melee or agility modifier for missile, plus any class attack bonus).
--add a bonus based on your weapon of choice (+0 unarmed, +1 for dagger (D4 weapons), +2 for swords/axes (D6 weapons), +3 for D8 weapons, +4 for D10s, +5 for D12s, etc etc).
--subtract target's Armor Class (+ defense bonus if any) from your total Attack Roll (die roll + all bonuses); the difference is how much damage the target takes as a result of your attack.
-Defend:
--add your die roll to your AC OR a target's AC (you protect an ally rather than yourself).
-Other:
--resolve all other actions as you normally would, spell check results, etc.
Stats that need to change as a result of these rules:
-Base Armor Class is 8 rather than 10.
-Armor bonuses add to your Base 8 AC, add Agility Modifier, etc (as standard rules).
-Keep hit points lower: PCs gain a static number of hit points based on their hit dice (D4 = +1 hit point per level, D6 = +2 hp/level, etc, like weapon bonuses).
-Monster stats stay mostly the same, except Attack Bonuses, Armor Class and Hit Points:
--Attack bonuses are based on monster hit dice: example 1, 8D8 Monster = +11 to attack: +8 (level 8 monster) +3 (D8); example 2, 2D4 Monster = +3 to attack: +2 (level 2 monster) +1 (D4).
--Armor Class is -2 (to reflect Base 8 AC).
--Hit Points
Not sure here if they need to be modified, but I feel like they might need to... Will have to playtest this and find out.
SPECIAL
Working together to target the same monster might mean additional attack bonuses or combined attacks. I think that would be pretty cool, but again, it needs playtesting.