Well here's the thing, fallout has a set amount of creatures, I'm a fallout 2 veteran and I liked most of the new creatures, but if bethesda creates 50 new creatures with at least 3 varaints to each I would hate them for it.
One or two new creatures would be fine but 10 or 20? That would just be pushing the lore over a cliffedge.
Next thing is: THIS ISN'T OBLIVION!!!
I don't know too much about the lore in Oblivion but it's a fantasy game with alot of fantasy creatures who live in a fantasy world, meaning they could probably put whatever monster they create in there. They can tweak or bend the universe at will.
Fallout is based on the real world, alot of animals died, extinct.
And some of the animals basicly stayed the same.
While some mutated into something bigger.
And Deathclaw's and Molerat's are from the Enclave's(right?) or Pre-War US's testlab's which were gonna be used to attack china with.
So they were produced by humans, not mutated over time like giant bugs.
And what animals live in the nearby area of Washington DC? (for example)
We need to think about the real world, for example, an elephant cannot be in DC. Why? Becuse Elephants live in some parts of Asia and Africa. A zoo? No, 200 years after a nuclear holocaust there ain't no way in hell that elephants survived from a zoo.
Cat's are extinct, horses are extinct. Many other animals too.
Do ya understand?
They can't just put any crap in there just becuse they feel like it.
And I woulden't want them to either.