Ideas to fill up the space?

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Definitely windows, with long fancy curtains. I've never seen a ball room without windows, it's usually one of the brightest lit rooms in a castle. You need at least one side of the room to consist of nothing but huge windows, imo! Right now it looks like the room is right in the middle of the building with no wall that faces the outside.
You could also place some sort of little stage for the musicians, with the piano on it and some decorative elements around, to emphazise the "ballroom" aspect more.