If Aliens and Predators were added, with a more complex AI it'd be very challenging and fun. The predators especially, having them use actual tactics instead of just charging in would make it so much more challenging to face up against.

I was really disappointed to see that Centaurs were no longer in the game.

In the Fo4 artbook, there are some concepts for Radbats and Radstorks. Shame those didn't make it into the game.

I would've liked to see the Enclave in the Commonwealth but that doesn't really count as a creature. Since they added cats I would like see some psychotic mutated cats.

A alien like monster. Something that stalks you for a long time would be sweet (Especially in a cave or even a Vault)

Seeing as the institute experimented on animals, it leaves room for more failed experiments and mutated monstrosities.

This fucker from Fallout 2

Have any of you seen the Aquatic Bestiary mod on nexus, not new animals but at least something is in the water now.

more mutated sea creatures would have been nice like whales or sharks or even a giant mutated octopus thing

More mutations. I am a huge Metro fan, and I love how gnarly and... Well scary they are.

The Demons are a beauty (bat winged Deathclaws? Hmm). Scary as hell.

Nosalis, and Watchers. Mutated moles and rodents. Bigger, hungry, usually in packs.

Spiderbugs, nocturnal. Mutated spider-scorpions? They're disgusting.

Mutated Crows (For the realism!) Why are the birds unaffected what so ever?

Anomalys could be interesting. They are in S.T.A.L.K.E.R as well I do believe. Could make the environment a little more interesting.

I prefer mechanical i like the theme of facing advance tecnology vs humans

I wanted to see more mutated insects in general, like giant bees, spiders, dragonflies and so on.