The sail passively gain energy from the current, so an upgrade would be making their own currents.
 The ship it self could do with an engine that manipulates the gravitational and EM fields around it. While it might not boost it speed, it might increase control and agility.

 The thing for higher speeds, can a wooden ship endure the strain. A metal ship would be difficult due to the need to cover up all iron and steel. Is it me or is metal more available than wood.

It is said iron messes with the never never due to sheer reality of it, so I wonder if it rots here due to a similar reason.

They could be a magical created version of stalagmites and stalactites but in this form of the spire. The material needed drawn from the very mists surrounding it, easy bet that those mists are not just water, they are sure to contain something unique, and the spires leach the stuff for its own form.

What if the spire was grown with a blue print imprinted? So it would not need continual intervention as it already knows what to become. I wonder if all the spires have the same basic design, it only differs in how it was outfitted and altered.

The silk has its own unique properties, so it would seem does wood. But only in sufficient quantities. And it need not only volume but it needs to be imbued with emotional energy. It can't be mass produced, their creators and careers must invest time and energy to make it so.

What if it was some kind of self generating material, feeding on something to increase in mass and quantity?

Wookies are kind of giant squirrels, I could see a feral cat population on the surface. I wonder if the is a sentient population on the surface possibly good possibly evil or simply neutral.

I wonder if we will hear about the stories the captain is thinking of? Stories often carry pieces of lore in the books.

I totally see Grimm becoming an etherialists, his ship acting as his personal crystal and it filling the gaps that open in him.