So a few questions. I'm assuming you liked Grayhawk Initiative, but had some issues with it. This isn't enough to tell me specifically what you didn't like (although I can make some guesses).
Also, a nitpick, but you don't get an Attack of Opportunity
and a Reaction. You can use your Reaction to make an opportunity attack.
So, the ultimate issue I have with Grayhawk Initiative, is the complication added to try to work in movement and bonus actions. In this version specifically, not all bonus actions are triggered by an action or movement. Many are independent, others are used before the action or movement.
You really have 3 states:
1d6;
2d6 if you want to move and act, or move and bonus action, or act and bonus action; or
3d6 if you want to move, act, and use a bonus action
This still penalizes people with bonus actions. It would penalize them less if you used a higher die type, since they could still roll a 1.
This doesn't take into account speed factor, which seems to be one of the intentions of Grayhawk initiative. That may be intentional on your part.
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So here are my thoughts on the system as a whole (Grayhawk/Gandalf). First, it doesn't solve the biggest problem I have with RAW in that movement is still tied to your action. I can't stand the idea that you can move 90 feet in a turn (Dash + Cunning Action), and attack (Charger Feat) while everybody else is standing still.
Second, it doesn't really work with bonus actions well. This also means it's going to have difficulty with legendary and lair actions too.
If we look at RAW, on your turn you can take an action, move, and potentially a bonus action, in any order, or combination of orders. I think that should be maintained.
What is Grayhawk Initiative trying to fix then?
Add a little more variety to initiative. That's easy to solve. Roll initiative every round.
Account for the time it takes for different types of action. You've removed the types of action modifiers (using different size dice that Mike used), and simply altered the initiative order a smidge by penalizing people who will use their turn more completely.
Make it easier to follow, by counting up from 1 instead of backwards from whatever the highest number is.
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So let's look a little deeper.
1. Everybody rolls every round. I like that (although some DMs don't because they have to roll for all of the monsters).
2. We should maintain the concept of a Turn. Your turn includes your action and potential bonus action. It also includes move (something I'd like to change).
3. We want to account for different actions, or potentially multiple actions, might be slower.
Declare first makes sense to me. If we consider that we aren't measuring when we start our turn, and instead when it resolves, this makes some sense. For example, if you declare I'm going to attack the orc with my sword. Then you are attacking with your sword, and when your count comes up in the initiative, you just happen to be connecting. This perspective doesn't work 100% of the time, but then neither does the other perspective.
So some actions might be quicker (casting a cantrip, using a light or finesse weapon) than others (using a heavy and/or two-handed weapon, casting a high level spell). So using different dice makes sense here.
d6 for light or finesse weapons, casting a cantrip to 2nd level spell
d8 for normal weapons, casting a spell from 3rd to 5th level
d10 for heavy and/or two-handed weapon, casting a spell from 6th level+
Without any modifiers, any of these can still beat any other one. But it's more likely that the quicker attacks will happen faster.
So what about taking a bonus action? There are several types of bonus actions. Bardic Inspiration doesn't interact with your action or movement. Cunning Action provides additional movement, but it is also not triggered by anything and can be used before or after your action as well. Some fighter maneuvers use a bonus action instead of one of your attacks (Commander's Strike), or before your action (Feinting Attack). I can't think of any that are triggered (that would be a reaction). So you have to be able to use your bonus action when you take your action. Personally, I don't think there should be any modifier for these. They are a
bonus action. Just like a creature that has multiple attacks shouldn't be penalized as to when the first attack occurs.
So what about movement? Well, frankly I think it's a problem already. Because clearly if you're going to move 30 feet before attacking, your attack will occur later. Because your movement is tied to your action, you are standing still while others resolve their actions. It seems logical to add a penalty if you're going to move, but only if you're going to move
before your attack.
So let's add a movement die. A d4. If you're going to move first, roll the d4 and move. Once you reach your count, roll the appropriate die to determine which count you get your turn (action + bonus action). If you attack first, then roll the d4 for your move after your action.
So, here's the new initiative for you:
ILBRANTELOTH INITIATIVE
(The simple version)
A Turn = Action + any potential bonus action.
Initiative (start at count 1, lower is better, no modifiers).
d4 for move (roll first if you move first, roll second if you move second)
d6 for light or finesse weapons, casting a cantrip to 2nd level spell
d8 for normal weapons, casting a spell from 3rd to 5th level
d10 for heavy and/or two-handed weapon, casting a spell from 6th level+
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Notes:
Spellcasting: Spells that end at the start or end of the
target's next turn don't change. Spells that end at the start or end of
your next turn end at the start or end of the same count in the next round.
Change Mind: If you change your mind, your suffer a penalty of the difference between the initiative you rolled and the new initiative die type. For example, if you were going to cast a cantrip and decided to cast a 5th level spell instead, you would go on your count +2. If you decide to cast a 6th level spell, it would be your count +4.
Delay: If you delay your Turn, you go at the end of the count that you select + 1d4-2 (minimum 0).
Ready: If you ready, you interrupt the turn (go at the start) of the count you select.
And a recommendation inspired by another thread:
Other (non) actions:
Free Actions: Certain actions take no time at all. Dropping a weapon for example. You can do one of these actions with each of your hands each round without penalty. To do a second such action requires you to use a bonus action, and a third such action requires your action.
Bonus Actions: Other actions take a bit of time, but not enough to cost an action. Bending down to pick something up, for example. You must use a bonus action. If you want to do another of these types of actions, it uses your action.
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Here's my slightly different version with no rounds. Although after this I might change it as well...
https://docs.google.com/document/d/12xPT-45PCBvaAJ3FaKI2Tkz2UmMGzoFcCzG0i7BfOss/edit?usp=sharing