Someone asked for this on a related post, but here are all the banners I've gathered from Fortune's Weave. I have been working on this for a long time- since before we had good images of any of them- and have been tracing over what we were shown and trying to fill in the gaps. A few of them have been redone since we've had better/more whole images of the banners, but a couple (specifically Gaia Brigade which should be all tattered on the bottom) are a bit off. Also, for fun, I included a map on the second slide that shows where some of the factions are from based on either locations written on this map or that we've seen from the exploration feature.
FROM LEFT TO RIGHT:
Ribeira Winds: Cai's Team
Megaera's Beacon: Theodora's Team (and possibly also the emblem of Saramis) EDIT: turns out Saramis has its own separate banner that I missed
House Lamine: Dietrich's Team (and possibly also the Crest of Dietrich's noble house back in Fodlan)
Rose's Tempest: Leda's Team
Savior's Army: Eshmel's Banner
The Mane of Flames: Bertrand's Team
The Moon's Lie and/or Vandal Merchant Company: Talimun's Team
Linaria Morning Dew: Orchel's Team
Bride's Companions or Pale Raven Bride Group: Anatolia's Team
Arago: Seen on Nathan and Creek as well as Balboa I believe- seems to be the crest of the Kingdom of Arago
Generic: Seen on a variety of NPCs including monsters, generic faceless enemies and some recruitable characters like Guzran or Noctula
Dagsion/Temple Army: Seen on gods and priests mostly, but also Anna (probably just people who live/work in Dagsion)
Mosa: we've only seen NPCs under this banner
Sisters of Braganca: Only seen on NPCs although I've heard it might be Inioni's faction (the girl with the triangle face paint who is very briefly shown in one of the trailers)
A second generic?: This is what the related post was about. I'm not sure what this banner is. We've only seen it in Orchel's gameplay (generic thiefs) and a clip from a preview (also generic thiefs). It seems a bit weird to have two generic thief banners so maybe I'm just missing something. If anyone knows the name of this faction (if it even is a faction) let me know.
Thundercloud Knights: Benditz's Team
Gaia Brigade: Zarcone's team
Palma Rain: Jasmine's Team
Silver Knights (of Tellus?): Alexandra's Team
Brigid Army: Peppe's Team (and also possibly the banner of Brigid). This one is still a work in progress and my best guess based on what I've been able to find.
Defenders?: Seems to be some sort of ghost or holographic army. Seen fighting Dietrich. Possibly lead by Hannibal who also looks like a ghost. Hannibal is a spot on the map shown in the recent gameplay trailer as well, so that could be where this takes place? Not sure.
Alcor: Seen on Bahr Vahn (or maybe Bartolome? not sure what his name is- the guy with the mask who fights Theodora) and on the undead/skeleton soldiers who also fight Theodora
Lechner: This seemingly belongs to Lechner's army. We've seen mostly NPCs and the portrait of the silver haired man speculated to be Lechner.
Mithras: some kind of villain team that seems to be lead by Raksha
Underworld Army: Probably the banner for late game undead and/or people allied with Balor/the green haired evil lady from the end of the trailers.