I don’t think they’ll ever add trading, personally. I’d they did, people would complete their Pokedex in no time, so they’d have to restrict it in some way, such as by making it so you have to be within a certain proximity to trade. However this might put people in risk from stranger danger, and I’m not sure Niantic would want that liability.
I suppose they could restrict it some other way, like by making a new currency (item) that’s hard to get and required for a trade, but then people would likely be upset that it’s too hard to get, or they’d complete their Pokedex too quickly. Obviously completing a Pokedex quickly would mean a lots of customers for Niantic. So I don’t think it’s in their interests to enable trading.