For me it doesn't really matter what the game has you build there, as long as it challenges you to acquire the space for it. But if you want brainstorming: a large crafting station. A large harpoon gun to catch some objects farther out. A fast travel station (human cannon?). A wind turbine energy source. A drill that mines the cliff. I do like the idea of some special plant requiring the space, too.
The idea being any of these would be required for progression. Except maybe the fast travel, which players would build regardless.