Basically we only have one good source of information, that is the official niantic website and twitter feeds and what niantic employees say at conferences or panel discussions.
Then there is another source that has quite a track record, its the dataminers and what they find in the data after dissecting a new update. They sometimes manage to predict what comes next, which is not surprising because niantic has to prepare content updates by putting the data into the game some time earlier.
Everything else is speculation and rumours, and the track record is terrible. They never get it right.