I have been using Terragen 3 on a couple of TV shows and I must say
everybody is really happy with the results I am getting.
There are a few things that would help terragen integrate with other packages even better
1)Render the clouds with exr2 and the ability to deep composite objects in the clouds in nuke
so for example I could render clouds without hold-outs and then composite an airplane in the middle of it
2) The ability to export the clouds as .f3d files for import in other packages as Houdini, Realflow(and maybe fumeFx)
3) support for alembic
Also it would be great to see an upgrade to the water using this paper
4) Simulating Ocean Water
http://graphics.ucsd.edu/courses/rendering/2005/jdewall/tessendorf.pdfthis paper includes an algorithm to create whitecaps
I apologize if these topics were covered somewhere else, if they were please post a link.
Aldo Ruggiero