Hear me out. Mobs are incredibly boring. They only want to kill you an rarely interact with each other. Yes, dogs, iron golems, and ocelots have this attribute, but other mobs just sit there all DERP or attack you. I propose that some mobs interact with each other, making it more beneficial to have certain mobs around. Here are some I think of off the top of my head. Disclaimer, not all of these are my idea.

Zombies should be scared of iron golems

Bats should be tameable, like in a mod I saw, and scare away spiders like ocelots scare away creepers

As said before on this subreddit, skelies should be scared of Dogs and wolves.

Most passive mobs should be scared of aggressive mobs

These are just some ideas, as I said, that I could come up with off the top of my head. Post in the comments if you have more.