Great thread idea ! Here are some more :
Hum3d car render challenge: only few days left ! but it happens yearly
https://hum3d.com/car-challenge-2018/
Michelin Design Challenge: quite more advanced and very competitive though:
https://www.michelinchallengedesign.com/the-challenge-for-2019/timeline/
Turbosquid 3d Art Competition:
https://www.turbosquid.com/3d-art-competitions
Artstation – Feudal Japan
https://www.artstation.com/contests
Weekly CG challenge:
http://weeklycgchallenge.com/current/
