You might put a desk in there, similar to a desk-under-bed like this kind of thing http://www.ghoofie.com/bedroom/modern-and-cool-bedroom-desig...

A shrine to a dead relative or a place for similar artefacts such as arranged cut flowers.

Futon storage, maybe. Depends on the type of flooring.

seating, storage, spare bed, play area for kids, etc. and combinations thereof