During combat, creatures do not take their own separate turns. Instead, they can act anytime during the round, and if an effect specifies an amount of turns, there are as many turns in a round as there are creatures. If a creature were to attack another creature and that creature has a usable attack, that creature can make an opposing attack roll. In an opposing attack roll, the attacked creature rolls their attack with the normal properties of that attack. If the two creature's rolls were in 3 digits of each other, both attacks hit, even if the Armor Class was higher. A creature cannot make an opposing attack roll against an opposing attack roll, unless explicitly stated so. Did I just steal that from the Sonic homebrew system? Maybe. But I'm probably just going to have these two rules for any free-flowing homebrew system. Each round, a creature takes (1 regular action) + (1 Inhaling/Exhaling action), moving freely throughout the round. Inhaling grants new copy abilities, while exhaling removes them. Oh, and also, there's the Block action (regular action). Reduces damage taken by 33% for 1 round, but decreases Energy by 1. The Rest action makes the user Unconscious for 1 round, but increases their Energy by 3.

Energy Bar

The Energy Bar starts at 0 and goes up to 10, maximum. Doing various actions fill the Energy Bar for stronger actions.

Energy

Inhaling/Swallowing; +1 every size above Small

Copying; +1

Flight; -1

Blocking; -1

Resting; +3

Items; +varies

Copy Abilities

You can have up to 1 copy ability at a time, which you can use to embellish other actions. 

●Tags; Tags are minor modifications to actions, not costing any Energy. (Ex. Add Fire tag to sword swing, making it shine dim light.)

●Enhancements; Activated by spending Energy Bar points, these can be used as standalone actions or something to be added to actions.

When a creature takes damage equal to 40% or more of its maximum health, it drops its copy essence, if it is using a copy ability.

Spells to Energy

Cantrip; 1

1st Level; 2

2nd Level; 3

3rd Level; 4

4th Level; 5

5th Level; 6

6th Level; 7

7th Level; 8

8th Level; 9

9th Level; 10

Combat

During combat, creatures do not take their own separate turns. Instead, they can act anytime during the round, and if an effect specifies an amount of turns, there are as many turns in a round as there are creatures. If a creature were to attack another creature and that creature has a usable attack, that creature can make an opposing attack roll. In an opposing attack roll, the attacked creature rolls their attack with the normal properties of that attack. If the two creature's rolls were in 3 digits of each other, both attacks hit, even if the Armor Class was higher. A creature cannot make an opposing attack roll against an opposing attack roll, unless explicitly stated so. Multiple active abilities can be used at a time unless something within one of the abilities contradicts another one, just remember that would (most likely) put all the abilities used on cooldown. Instead of spell slots, after using a spell, spells of that level go on a round recharge. 

Cantrip; 2 rounds

1st Level; 3 rounds

2nd Level; 5 rounds

3rd Level; 8 rounds

4th level; 10 rounds

5th level; 13 rounds

6th Level; 15 rounds

7th Level; 18 rounds

8th Level; 20 rounds

9th Level; 25 rounds

For Warlocks, since they cast spells at their highest costing available slot/recharge, can cast spells twice before going on a round recharge, just not cantrips. Also, for other abilities not mentiond here that say "usable x amount of times before short/long rest", they will be given appropriate round recharges. The free-flowing round system, opposing attack rolls, ability combinations, and spell charges are meant to incentivise teamwork and creative team-up abilities, for both sides, as well as blurring the line between combat and roleplay. Oh and also for cool anime moments. Or smth. Because that's cool. Resistance In this system reduces ALL damage taken by a certain amount. 

https://www.dndbeyond.com/sheet-pdfs/LewelKMW39_157043221.pdf