2. Burrowing- Mole rats, giant rats, radroaches, or other appropriate creatures could surprise attack through burrowing from the walls, celling, or ground. This could make these creatures have a very different encounter feeling.
3. Crawling on the ceiling and the walls could also make encounters very interesting. Just picture 20 radroaches coming down a hallway at you, crawling over everything, falling from the ceiling on you. That would be a very fun way to use low level monsters at high levels.
4 Giant monsters like Liberty Prime should be rare, but at high levels each player should have an experience with them.
5 Pack monsters like dogs, wolves, and deathclaws should use pack AI when in groups. When a pack animal hunt they stalk their prey and surround it. They don't bum rush it.
6 Bearmice- mouse the size of a bear
7 Giant spiders...really no giant spiders...we need giant spiders
8 So we have brahmin and bighorners, but what about horses, camels, bison, deer, or whatever else. These could be used as wild animals and pack animals.
9 Cyborgs- Now some people will say these don't quite fit, but with the tech developed by places like the Enclave and the Commonwealth Cyborgs should be a semi-rare opponent for high level opponents. They could be modeled after ghouls. There could insane cyborgs who are malfunctioning and lucid ones who could be centuries old or brand new.
10 Robots with missions. The majority of robots should actually be doing something besides roaming the waste attacking things. Good examples of this are the subway protecterons from FO3 or the Mr Handys at Repconn headquarters.
Thats my rant for now. All I want for Christmas is APA and a gauss rifle.