I proposed a thread where people who want help would ask for it, based on specific audio. (Mixing or non mixing issues)
There are multiple paid and free web sites that offer exactly what you are proposing. One of the websites,
+++ Mix Challenge +++ accepts audio files for use in the contests.
+++ Modern Mixing +++ offers prizes for winning mixes.
Another forum I'm a member of has a section VERY similar to what you're requesting. The Cakewalk forum "Song" section is very well known for the critiques members provide.
+++ HERE +++ is a sample thread.
In my mind, the public face of the forum is more about encouraging others and praising effort than it is about critiques. I do know that many members private message other members with ideas and analysis "off the record". But even then the messages are gentle and offered only after each member feels a personal connection is established.