FOSS gamedev and creative worlds

Is it possible to use a better player model that can turn his head and bend knees for example?

Not really, until client-side scripting is supported. There's currently mods that do what you want, but they update on the server and are incredibly laggy and slow.

Well kaeza, did a mod about turn the model's head....

But the bend knees, i don't know if exists a mod about that...