"I am not in favor of WIP during the contest.
Some people might feel "scared" by some users right at the beginning of the contest just by viewing the wip of some works."
I read that and all I could think is: Life is tough, buy a helmet. It's supposed to be a fun learning experience for everyone, so if you can't handle a friendly competition, how will you handle the real conflict that is constant in adult life. Maybe a snowflake contest next year to accommodate that?
I see some of the stuff here and I think "Wow, I never, ever would have thought of that. Nice work." Not "Give up and run away." I expect to get clobbered by people far more creative than myself here, and I've been in the VFX trade for nearly 20 years.
Photogammetry terrian should be excluded, but I'd say you're free to use imported plants, props, images, all of that as long as the terrain, water and sky are all generated within Terragen's procedural engine. Wisdom comes from experience, and experience comes from bad decisions. Sack up, experiment, and have fun.