Each new mob created each year seems to have better animations than the older mobs, therefore I would like to suggest to remade the animation of mobs that have been added during Minecraft's early days. You could just slightly tweak the animation of these mobs to make them have more movement to their models. The reason I suggest this is because I notice that the animation of old mobs are stiffer than the recent ones. Though I understand that revamping the animation of old mobs might result in flexible animation which will ruin the Blocky theme of minecraft and make them look like out of place but if done in a right way, could give more depth to the game. The mob's animation in Minecraft Dungeons is a great example of how to revamp the animation of these mobs.Besides that, to make the survival aspect of the game more intense you could add sprinting animation to some mobs like creepers.