There are a lot of problems associated with trading and I see no way they can ever be addressed.
The main one that would concern niantic is it would hurt their profits, why would I keep buying incubators to hatch 10km eggs for a lapras when I could just trade for it for free?
I think they will continue to say trading is coming but will never implement it.
Side note:I highly doubt PvP will be implemented either. They have 70 employees, I doubt they would focus their attention on such things that would have new bugs to fix every week and make the framework to connect players in the first place.