If you are looking for some new and unique trees to give your tileset, module, or PW something fresh and unique, and if you aren't afraid of converting OBJ's into Max or Blender to convert to NWN, I encourage you to play around with
tree(d) from frecle. It is a very lightweight install that works crisp and fast.
I haven't gotten as far as getting them into NWN, so I can't speak to how easy that will be or good they will look in NWN, but the tree options are very robust, and the textures they provide look good on their models. You can preset a variety of parameters that dictate the number of branches, sub branches, the leaf propigation, branch and sub-branch angles, curvature, and kink. Once you dial in settings you like, you can randomize the seed, and it will give a new variation of the template you spec'ed.
By wildly tweaking those settings I was able to create wild variety, including numerous tightly branched vertical growth like an
Italian Cyprus, Thick sparse twisting branches like a
Joshua Tree (which would have needed a custom texture to be perfect), and my most interesting find while playing with the settings, was a tree with wildly inverted angles and curved branches and sub-branches, giving the leafless tree a spiky spiderweb effect that was disturbingly unnatural... my skin crawled as I looked at it.
I have read in stories about trees or buildings with unnatural angles and so forth that disturb the viewer, but I had never actually seen such a thing, until now.Regardless, I am hoping that somebody with more NWN Max or Blender experience has a chance to play with this before I get that far, even if it is just to shoot it full of holes. Good Luck!