I am open to many different styles.
I have ideas about a cover showing William's pistol, which is a breach loading silver flintlock with ivory inlay in the handle and wild flowers etched along the barrel's edge.
I also had a thought that the town of Blackbile could be depicted on the cover in more of a painted or silhouette style. The town is on the slopes of a volcano at the edge of a mountain range and there is a river running through it. The town itself looks like an American frontier town with wooden buildings, muddy roads, and wooden boadwalks. Other important landmarks include a brick-built town hall with a dome on the roof and a black chapel with tall spire and upturned cross. There is also a square bell tower with spiky fortifications on the outside.
Just as another detail, it often "snows" ash in the town. Guns fire bullets in paper catridges. Blood splatters could be good.
The assassin's guild logo is intertwining cresent moons that form a sun, this may be good on the spine.
Though I am adverse to characters' faces on the cover, there could be part of a person or someone facing away. William wears black and brown linens, has blonde hair and is caucasion, average height.
I am open to both a traditional painted/illustrated looking style or a stylised pop-art minimalist look. (I am not really up on all the terms but I hope you get the idea.)
Please feel free to come up with your own ideas and don't feel you have to stick rigidly to my ideas.
Below are some more details that you may find useful (Some may be repeated from above)
The city of Blackbile is on the slopes of a volcano. It is a typical american style frontier town (think cowboys) but with more eurocentric landmarks. There are wooden buildings with wooden facades, muddy roads for horses carts and carriages. There is a wooden boardwalk for people to walk on to keep out of the mud. It often "snows" ash in the city, so most places are dirty and grey.
The notable landmarks of the city are the town hall withis a large brick built building with a dome on the roof. A large chapel/ cathedral with upturned cross at the to of the spire and another building that is a square bell tower. A river runs through the city called "The Landslide" it is choked with dirt and rocks and ash and looks like flowing earth.
William is slim, average height. Short blonde hair. generally wears brown and black linens, basic shirt and trousers. Has a red scar on his inner forearm that looks like a red smear (left). Weilds a silver flintlock. He also has the assassins guld logo branded on his arm. (intertwined crescent moons that make a sun.