Is your feature request related to a problem? Please describe.
Player models are quite limited, and it is basically impossible to even attempt to imitate the animations of for example, the Minecraf ttrailers.

Describe the solution you'd like
It would be nice to have way more variables for player models, like flying, some interface to the item and its tags that the player is holding, having on as armor, etc (no idea how you'll do this, maybe dynamically adding variables for every single tag and item and item slot that exists? add some special syntax?), what biome the player is in, etc.

Does OptiFine do this
No. Player models are an EMF feature.

Additional context
None.