I'm on Book 2 of Carrion Crown and the party is gearing up to take on the Trial of the Beast. I'm thinking of once they have taken care of that, an opportunity for one of the PCs to join the Order will open up (one of them is a noble with a cryptic grimoire, so probably her) and I'm thinking of how to make this group seem really important and cool.
I'm thinking doing things like finishing up the mess in Ascanor Lodge, and the cult in Illmarsh would get them a butt load of sway in the organization, but I'm thinking of what can the party get in turn for helping them and ascending the ranks? I've read the Occult Mysteries and I think that is a good start, providing services depending on their prestige. I'm thinking of just adding other services that can be called on. I made one NPC that is a level 10 witch that is effectively a errand girl for the Order, so they can call on her with special tokens and they could have her run errands and provide divinations for them (also have a small subplot with her). Also since the Caliphas chapter seems to have a room filled with exotic things, what if there was a heavily guarded box with contained a Harrow Deck of Many Things, and you could use your pull in the order to potentially draw from it? Any thoughts or additional ideas for this?
In terms of rewards, I'd personally choose anything that seemed like an Ustalavian Artifact/Item of history (With the stipulation that the item must be returned back to the Esoteric Vaults in the same condition it left),
Or anything that is Osirian in nature. So, time to crack open that Mummy's Mask book, or anything the Pathfinder Wiki says is a source book for Mummy's Mask.
I'd suggest looking at the Faction Guide for more concepts as to how they could get more sway and how they could spend said sway, or at Lost Treasures for ideas for powerful items the Esoteric Order could have.