I'd love a return of the extendable wooden posts, where you could stack fences on the top of a post to make it taller. Another option would also just be double height posts as separate items.

I second this. Imo, the best way to do this would be allowing post extension with fences (treated wood, steel, & aluminum) instead of just a double-height version, as the former allows users to create the pole height that best suits their needs.

Immersive Posts is an add-on that does this, and even allows you to use more types of fences as posts.