I’ve seen it speculated that the character on the left of the banner art, with the flamer, is probably some sort of priest or sub-class of them.
However, while flame weapons are something they have been mentioned in the lore to make use of fairly often, that character doesn’t really have any of the main features you’d expect of Imperial Priests. If you take a look at this page here and at the various other types mentioned on that page, the trait they all have in common is some form of robe or similar garment, typically with a hood, even the battle-focused Drill Abbot does and so does the priest character revealed in the game so far. All the official Miniatures I can find find of them also have them along with the usual assortment of iconography, scripts etc. Outfits tend to be either simple, or highly elaborate. The character there doesn’t look like anything like any of the official artwork or miniatures for Imperial Priests that I can find.
Instead the character on the left of the banner artwork is someone who appears to relatively well equipped, if you brighten up the image it seems they have both elbow and knee pads, some sort of high-tech headgear, pouches, there’s 2 different materials at their shoulder so either some sort of shoulder pads or perhaps chest armour with an under layer etc and they don’t appear to have a robe or any iconography that we can see beyond those purity seals. They also appear to have a sword or something similar, and a strap on their flamer, so they’re someone who’s outfitted for battle in a reasonable way, just not in a typical militaristic Imperial Guard style.
One character type that commonly features as part of Inquistorial retinues is the warrior acolyte - basically just an Inquisitorial Warrior/Soldier - who can be outfitted with a wide variety of different gear, styles, weapons and such. Here’s the miniature for one of them, although there isn’t really a defined look for them and they can vary a lot. https://www.games-workshop.com/resources/catalog/product/920x950/99060108075_WarriorAcolyte1NEW01.jpg
Tthe basic aesthetic style between that miniature and the art seem broadly similar in terms of outfit type (While the character in the art doesn’t have a full robe, they both have outfits that fit the idea of “Inquisitorial Warrior” rather than being just a typical soldiers gear) the flame tank on the back, similar headgear etc. The characters in the game are obviously not meant to be a copy of a specific miniature exactly and it seems like a suitable analogue for “Warrior Acolyte” in the art. Technically classes like the Guardsman would also count as “Warrior Acolyte” but I’m referring to the actual miniature as an example for the sort of thing a ‘typical’ one might be like.
The character on the right with the cloak is something that the most common guess I’ve seen for is a scout of some sort, but what they’re holding appears to be some sort of melee weapon with a grip low down on it, and it seems like the ‘cloak’ might be something that has sleeves as a gloved have can be seen sticking out, and those shoulder straps would make it difficult to use if it was just a freely-draped cloak/cape style thing like the Cadian sniper miniatures have. The other guess I’ve seen has been some sort of Psyker, in that case i’ve not been able to find any Psyker art or miniatures with a grip on their staff what they’re holding doesn’t appear to be very staff-like. That cloak could just as easily be a robe, and what they’re holding being a power hammer - the weapon the Priest in the trailer was seen using most often when they were shown.
Obviously the game will have customization options so features won’t match exactly, but to me both of them seem to broadly fit those classes - the character on the left as sort of warrior acolyte (Perhaps a Pyroclast, who focus on that sort of thing) rather than them being the Priest, with the right character being the Priest with a hooded robe.