As much as I would say that I like to see more scary monsters, you have to keep in mind it still has to be in the same atmosphere as the rest of FFXIV. I rather say I like to see an expansion of the already created bestiary especially Beast Tribes, Wavekin and Ashkin.
Maybe later on they can introduce a new category of beastiary along with the Garlean Empire. Like sort of mutant monsters or tamed beasts. I also think they should focus the higher lvl monsters to become more monster like and less cute or innocent looking.
What I personally like as monsters are:
  1. Topielec or drowners
    The topielec are spirits of human souls that died drowning, residing in the element of their own demise. They are responsible for sucking people into swamps and lakes as well as killing the animals standing near the still waters.
  2. A bukavac would be quite cool for higher lvl lakes
    Bukavac was sometimes imagined as a six-legged monster with gnarled horns. He lives in lakes and big pools, coming out of the water during the night making big noise, jumping onto people and animals and strangling them.
  3. Lady Midday or Pscipolnitsa, I would find such a thing awesome to see between Gridania or Thanalan or a similar but higher lvl area.
    Pscipolnitsa, who makes herself more evident in the middle of hot summer days, takes the form of whirling dust clouds and carries a scythe or shears, but it is likely that the shears would be of an older style, and not akin to modern scissors. She will stop people in the field to ask them difficult questions or engage them in conversation. If anyone fails to answer a question or tries to change the subject, she will cut off their head or strike them with illness.
  4. Baba Yaga a special NM mob for Gridania.
    She flies around on a giant pestle, kidnaps (and presumably eats) small children, and lives in a hut that stands on chicken legs.
I got this all from European especially slavic mythology. It could be a great source of inspiration for an original game.
I also must say I'm not at all an expert on Final Fantasy game, infact this is my first Final Fantasy game.

On a side note; it would be cool if humanoid enemies could actually talk or say certain things during battle.
And if certain animals would appear only in nighttime and others in daytime. But I'm aware this would be very difficult to implement now. But I would like it.