Hello there. Since the world of Equestria leaves a lot of details about it's world vague (for now) that gives us authors the wonderful opportunity to shape things however we want, especially since we're not limited by having to go through executives, standards and practices etc.
We have the world at our finger tips thanks to the FiM writers and I think it's wonderful that so many authors explore it in so many different ways.
So in a nutshell, that's what this group is for; worldbuilding, personal headcanons (from as major as entire backstories for the kingdom to as simple as what ships you choose to support), world building for AUs or world building you think could fit right at home in the show, developing OCs who exist in Equestria, just about anything you can think of.
I hope we'll see an active forum here, I love to hear about the worlds other people have crafted from FiM and the characters therein.
Have fun everypony!