VARIANT INITIATIVE SYSTEM
The intent of these rules is to hopefully add a little tactical complexity and crunch and unpredictability to combat without it being overly cumbersome. The general idea is that you can do all the things you could normally do on your turn, just not necessarily all in one continuous turn. Your turn may be interrupted by another combatant's turn depending on your relative initiatives and the kind of action you're doing.
Initiative is rerolled every round.
Players have the option of making an Int or Wis check to determine initiative rather than a Dex check.
Any action you can perform such as Movement, Attacking, Casting a Spell, etc. is associated with an Initiative Point Value (IPV) (see below in the next section).
On your initiative, after performing an action, you subtract the IPV for that particular action from your current initiative value to obtain your new initiative. Initiative values can normally drop below 0 into negative numbers after subtracting the IPV for an action without any consequences or penalties.
Then check to see if your new initiative is still the highest initiative among all the active combatants for the round. If it is, then you can perform another action that you may have remaining for your turn. Continue to subtract the IPV after each action performed and continue to check if your new initiative is the highest. Ties go to the combatant other than the one who just acted.
As soon as your new initiative goes below another combatants initiative, that other combatant gets to start their turn. After they subtract the IPV for their action, initiative is again checked and again the turn goes to the combatant with the highest initiative.
If you are now the one with the highest initiative you can continue with your turn subtracting the IPV for any remaining actions you perform. Your turn ends when you have no other remaining actions you can perform that round or when you choose to end the turn.
Continue in this manner until all combatants have finished all their actions for that round. Then advance to the next round.
Reactions, as usual, can occur anytime immediately upon trigger. The IPV for that particular action performed must still be subtracted from your initiative value (unless you're already done with your turn for the round).
Fiona the Fighter and Clark the Cleric are up against Troy the Troll. On Round 1, all combatants roll for initiative. Fiona gets a 15, Troy gets a 12, and poor Clark gets a 5.
Fiona is first up. She wants to close the distance with Troy to bash him over the head with her maul. Since Troy is 30 ft away, Fiona uses her Movement and moves her full speed next to Troy. The Initiative Point Value (IPV) for moving your entire speed is 6 so her new initiative is now 9 (15-6). Her turn is now paused and Troy gets to act because his initiative is 12.
Troy saw Fiona rushing him and so was ready for her. He uses his Action to multiattack with his bite and 2 claws hitting on 2 of his attacks causing significant damage. The IPV for his multiattack is 9 (3 x 3) which puts his new initiative at 3 (12-9). His turn is paused because Fiona's initiative is 9.
Fiona resumes her turn since her initiative (9) is the highest now. Trying to exact some payback, she swings her maul clobbering Troy over the head. The IPV for her maul attack is 4 because it's a heavy weapon. Her new initiative now is 5. Her initiative is tied with Clark's, so Clark gets to act because Fiona was the one that just acted.
Clark sees that Fiona is hurt badly and so uses his Bonus Action to cast Healing Word at 2nd level. The IPV for casting this spell is 4 (2 + 2nd lvl spell slot) which puts his initiative down to 1. Fiona is next to act again because her initiative (5) is now the highest.
Fiona resumes her turn and decides that she needs a little more of a pick me up and so decides to use her Bonus Action for Second Wind regaining more valuable hit points. The IPV for Second Wind is 3 (see Other Actions section) putting her initiative at 2. But since she has used her Movement, Action, and Bonus Action, and does not want to use any Free Interaction, her turn is now done.
Next up is Troy who's initiative is 3. He resumes his turn and decides that he doesn't like the odds now with Fiona going back up to near full health and Clark still yet to take his Action. So he makes a break for it and retreats his full speed. Since the IPV for moving your entire speed is 6, this puts him at -3 initiative. But now Troy has used all his actions and so ends his turn.
But before Troy gets away, Fiona uses her Reaction to take an Opportunity Attack hitting him over the head once again with her maul. The IPV for this action is again 4 but since her turn is over, this doesn't matter.
Example Scenario Continued
Now Clark resumes his turn with an initiative of 1. He sees that he can't reach Troy to hit him with his mace so he decides to cast his powerful Sacred Flame cantrip at him. A column of radiant energy fries Troy for surprisingly heavy damage. This action has an IPV of 2 putting him at -1. He would still be able to act since he has his movement left but decides not to pursue and so ends his turn.
Round 1 is now over and round 2 would start with rerolling initiative.
INITIATIVE POINT VALUE FOR ACTIONS
As usual, your movement can be broken up before and after other actions. However, the moment you stop moving or moved your entire speed, you must subtract the IPV for that movement before performing any other actions.
IPV for moving your entire speed is 6 pts. The IPV for moving a certain fraction of your speed will be the corresponding fraction of 6 pts. For example, if your speed is 30 ft, moving the full 30 ft is 6 pts. Moving 15 ft (half your speed) is 3 pts, moving 10 ft is 2 pts, etc.
Standing up from prone is 3 pts.
Attacks using weapons with 2-Handed, Heavy, or Loading property is 4 pts.
Attacks using weapons with Light or Finesse property or Unarmed strikes is 2 pts.
Attacks with all other weapons as well as Grapple and Shove actions is 3 pts.
Multiple attacks, for example from Extra Attack or Multiattack or a Bonus Action attack, and any riders associated with those attacks, for example many of the Battlemaster maneuvers or Paladin's Divine Smite, can all be made before having to subtract the IPV, unless you break up your attacks with another action such as Movement in which case each attack that was broken up in this way must first be subtracted before performing the other action.
The IPV for multiple attacks is cumulative.
Casting a Spell
The IPV for casting spells is 2 + spell slot level used (cantrip = 0 for this purpose). For example, the IPV for casting a spell using a 3rd level spell slot is 5 pts. Casting a cantrip is 2 pts.
The IPV for Dash and Disengage is 0 pts. However, the IPV for the movement associated with the Dash and Disengage Actions must be subtracted as noted above.
Minor Free Actions and actions that are just riders to other actions and not an action by itself are 0-2 pts (DM discretion).
All other actions are 3 pts.