Make AnimationTree more user-friendly

[FlooferLand]

Problems

First, lets say you have a 3D game and one main state machine that contains 2 other state machines, one state machine that handles idle/walking/running animations as well as another state machine that handles interaction animations, and you want to change how they are structured (moving 2 of the state machines inside another state machine?). This is currently impossible as there's no way to copy their data to the clipboard, and if you save their AnimationNodeStateMachinePlayback from the inspector or copy them to your clipboard, you won't be able to load them back in. As far as I've tested the only way to do this animation refactoring is to remake both state machines from scratch.

Second, state transitions are rather messy. You cannot have !moving as an advance condition, you must add another separate boolean called not_moving, or you could use expressions which lack any good documentation (especially in-editor) and require setting up variables in a script as well as setting advance_expression_base_node to the node containing those variables.
The prior is what I've noticed most people end up doing, and it results in a rather polluted inspector after only adding 4 animations:

There's also the problem of it generally not being user-intuitive, and the UI being very barebones and what I can only describe as a 3.x remnant. When compared to animation tree / state machine solutions other game engines provide, Godot's AnimationTree falls behind immediately.

• Unity has an "any" state that lets you transition to/from any state inside the state machine.
  Godot lacks this and requires manually creating a transition to/from each one, which can get very messy.
• Unity allows you to drag some animation states over a state machine in order to move them into that state machine, with Godot you have to delete those states and remake them from scratch
• Unity lets you create conditions at a top level on the state machine, statically defining the types and their default values, while Godot automatically generates them from transition conditions and is fundamentally unable to parse booleans used inside the expression field because of this.

The general solution to all these problems that I've seen is using several AnimationPlayers while manually creating your own animation state manager via script as a workaround.

Solutions

I lack the time / engine knowledge to implement these myself, but I hope this sparks a discussion to either fix the existing AnimationTree's flaws or fundamentally fix it's issues via a rewrite.

I propose 2 main solutions; Neither are are final, and I'd appreciate further feedback and more ideas as this is a discussion thread.

1. A new AnimationTree

This solution would most closely represent the current AnimationTree system, and this could either be a brand new node or improvements to the existing AnimationTree, though the latter could make it challenging when it comes to keeping backwards compatibility so I would not recommend it.

Here are some major improvements I've thought up:

1. A separate editing interface you can fullscreen

2. A new "Properties" panel on the left that lets you define animation properties as well as their types, giving you autocomplete for state transition conditions, and potentially letting you use inline GDScript?

3. Revamped interface; Allows copy/pasting of states, and dragging/dropping of states into other states (state grouping of sorts)

4. condition is fully replaced by expression.

   (When upgrading a project to the new animation tree system, the value inside condition could simply be moved into the expression during the project upgrade)

2. Animation scripts

This solution is more similar to Unreal Engine's animation blueprints, using scripts to define when and how animations get played.
I'm not sure about the actual API for this, but here's an example of what I mean:

class_name MainState extends AnimationState

const MoveState := preload("res://animation/move_state.gd")
const AttackState := preload("res://animation/attack_state.gd")

@export var player: PlayerCharacter

func _register() -> void:
	add_state("move", MoveState)
	add_state("attack", AttackState)

func _update_state(anim_system: AnimationSystem) -> AnimationTransition:
    var transition := AnimationTransition.new()
    if player.is_attacking:
        transition.name = "attack"
        transition.time = 0.0
    else:
        transition.name = "move"
        transition.time = 0.5
    return transition

class_name MoveState extends AnimationState

const IdleAnim := preload("res://animation/idle_anim.tres")
const WalkAnim := preload("res://animation/walk_anim.tres")

@export var player: PlayerCharacter

func _register() -> void:
	add_state("idle", IdleAnim)
	add_state("walk", WalkAnim)

func _update_state(anim_system: AnimationSystem) -> AnimationTransition:
    var transition := AnimationTransition.new()
    if not player.velocity.is_zero_approx():
        transition.name = "walk"
        transition.time = 0.5
    else:
        transition.name = "idle"
        transition.time = 0.5
    return transition

class_name AttackState extends AnimationState

const AttackAnim := preload("res://animation/attack_anim.tres")

func _register() -> void:
	add_state("attack", AttackAnim)