I’m a fan of Savage Worlds and always liked the card initiative system they used. Every new round of combat everyone draws a card and then act based on the suit and value of their card. It makes for a dynamic turn and some unpredictability when everyone acts. Best of all if you flub your draw, you aren’t stuck for the entire battle going last.
Mearls was playing around with an idea of doing something similar. However you’d roll dice and your planned action would also influence the type of die you roll. Initiative starts and 1 and goes up. Lined up a shot and ready to fire off an arrow? Roll a d4. Locked in melee and ready to swing? Roll a d6. If you have to move and engage an enemy, you are rolling additional dice to add to your total.
This way relatively quick actions like firing a bow or swinging a mace will probably roll low, but more intricate actions and those combined with movement take more time and are more likely to act later in the round. This isn’t necessarily the case however and a player (or monster) might still luck out and roll very low. But like drawing cards in Savage Worlds, if you tank a roll you aren’t stuck for the entire battle taking your action last.
Another thing I like about this is that players can plan out some, thinking about their actions and adding a little strategy to the battle. The more simplistic the action, the more likely you can act before the opposition. Trying to run around doing backflips and taking bonus actions are possible, but also mean you’ll likely act later in the round.
I borrowed some ideas from Mike Coleville having larger, more damaging weapons use a higher-faced die. I also split up spells so that higher level spells used a larger die to determine initiative. Cantrips were slower action spells compared to melee and ranged strikes, but could still be fired off faster than casting prepared spells.
The pickle with this system is that you can get a blown turn where the opponent you were going to attack becomes a casualty from another player. Effectively this would cancel your action for the round if planning on just making an attack. I decided that if your planned action gets cancelled due to no longer having a valid target, the player could always make a movement action allowing them to set up for the next round.
I am still struggling some with figuring out how the opposition uses this. Commonly I just roll a d8 or a d10, allowing the players to have more chances to act before them. It works out some as there are those rounds where most of the party gets to act before the monsters. While there are also those occasional turns where I luck out rolling a 1 and having all the baddies attack first for the round.
So far my players like this. Most cut their RPG teeth on Savage Worlds and not getting stuck with a single initiative roll for an entire combat is something they appreciate. There are still some kinks though and having essentially wasted actions (especially those making melee attacks) is something I found players occasionally grouse over. Another issue is there are feats and abilities that revolve around rolling for initiative, and players dumping into a high dex so that they can get that initiative bonus lose out some. I might tweak with allowing a single reroll per battle for every +4 initiative bonus. Regardless it seems to be a hit so far and you can find my tweaks for an alternate 5E initiative system in the downloads section.