I see different symbols in the world for example the black O on the wall in the second floor of Pieros shop or a weird symbol like the one you click on for the community forums, what are they?

The door in Piero's workshop is his "Door to Nowhere" correct me if I'm wrong but I believe he is trying to create it so people can travel great distances instantly (teleporting) some people think this might be used in future DLC.

That is a Portal. Even resembles one. I'm thinking it's a nod to Valve's Portal games.