Idea for simultaneous turns?

[SangoProduction]

I'm thinking of running a one shot where everyone rolls 3 dice at the beginning of the round. And then everyone writes down what they will try to do this turn, and then flag themselves ready when done. (They assign any of their dice to each of their actions that need one. Maybe add feats to increase die pool by 1 each.)

When everyone is ready, the actions are read off in order of initiative.

Obviously, there might be some logistics issues with die rolls. And occasionally some actions will be less applicable than others, once their turn comes. But any other problems I should expect?

[Fero]

I see two major issues.

First, doing things this way significantly increases the power of the Initiative stat b/c characters who go first will take logical actions but low init characters will take nonsensical actions (fireball empty squares, etc.).

Second, and relatedly, a lot of low init characters will lose actions. For example, the low init fighter who attacks the goblin may often find the goblin dead by the time his initiative comes up. Players
With low init characters may feel cheated, frustrated or bored.

On the flip side, it could be very tense and fun, especially with a smaller party. You may look at the game Diplomacy as an example of a game that uses a similar mechanic.

[Telonius]

Old-school NES comparison, but one of the earlier Final Fantasy games had a similar mechanic. They changed it in the next version so if your primary target was dead, you'd attack a different target.

"Frustrating" is a very mild way of describing the mechanic, and in the game you were controlling more than one character. Having your only controlled character in a TTRPG stand there like a chump is very much Not Fun. Losing a turn should be a nasty status effect, not an expected and regular outcome of playing a heavily-armored character.

[SangoProduction]

So, everyone should have a Contingency Action, that's broad and generally applicable, written at the start of combat.
This way, they can choose between the pre-written "I attack the nearest / most flanked / etc target with [weapon]," or their round-written action.