Your work is inspiring... Thought I'd throw a few ideas at you, not sure how written in stone any of your current stuff is, but...
With your waterfall its way too rigid. What I would do is build up the wall using small blocks. If you are looking for the manmade approach remember bevels are your friend... What I would do is try to push your wall back so that its more of a curve than a flat face. Both in nature and in man made dams there is a ellipitical shape to the retaining wall. I have an 5 minute sketch that will sort of illustrate what I mean.
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Atleast I think I understand what it is your going after.
The other stuff I was impressed with was your trees. I wanted to impart some of my own ideas about those, but hey... they are your trees so do what you want
. Remember that CYAN had the biggest probelm with trees in the original MYST game and there is a reason. Getting that organic look is NOT easy to do. IN fact its tricky. When I was in school years ago learning the ways of MAYA 3D, we cheated using nothing more than Nurbs and Artistic effects. Unfortuneatly Blender isn't so easily manipulated. Really the only way to improve the look of the tree is to ramp up tesselation... which will also decrease performace in the game... and it can decrease it alot. I think this might be one of the many reasons why URU only has one tree that has that organic feel to it. ELDER KEMO I believe it is... the rest look less than real. One idea would be to raise the trees up away from the users feild of contact. Putting them up on a hillside or cliff away from direct contact, that way you can take a mesh then put a really good quality PNG image of a tree. Its cheesy I know but it works. Otherwise you run into issues, atleast in my experiance.
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If you are dead set on having your trees look organic and have people play amoungst them then I would make them really big so that you only have 2 or 3 in a given area... maybe have more Meshes hanging off of them with vines and flowers to help hide the klunky spots.
/shrug
Anyways... If you find any of this helpful great, if not hopefully the post is easy to pass over.