With regards to the introduction of new things. In truth, Ive been disappointed with this aspect of the franchise more than Ive been happy with it.
The Queen was cool. The Dog-Alien, I loved. The stuff in A:R I liked, in concept, alone. The Newborn looked bad, Ripley 8 was cool if she was actually weirder and not shoehorned into being an Action-hero, the clones were cool (I really wanted to see what was cut out of Ripley 7), I hated the amphibian webbed feet on the xenos, the Predalien looked cool but made zero sense, the Engineers in the beginning of Prometheus are cool but once they wear those suits and have glowing holograms coming out of the Giger-esque tech, I hate them. Also, their suits would have been cool if they actually merged with their flesh and I got to see that, the snakes were cool, the Fifield-mutation was lame, the Trilobyte sucked, and the deacon was too big when it chest-bursted from the Engineer, it didn't look right coming out. I wish we had never seen a chest-burster in Prometheus at all. However, I liked it (the Deacon and Prometheus) overall.