Info is usually mined from the game files when it gets updated (not content updates) and then dispersed via Discord and Reddit and also Facebook.
As for "What banners are next," I'm using deduction.
New units are almost always released in accordance with patch order.
Patch 10.9 added Satoru, Hevijak, and Furiluga. We then got a guaranteed Dynamites, Dragon Emperors and Lugas.
Patch 10.10 added Yamii and Deth-Troy-R.
We just had a guaranteed Pixies so next (unless they change gears in a weird way) is gonna be Iron Legion.
Patch 11.0 (current for non-JP) added Sparrow and White Cyclops, so once we get through Troy-R, their banners are next.
And after them is 11.1, which added Lucifer of the Almighties banner.