I've recently purchased a home which has 5 bedrooms. While we only needed 4, I couldn't pass this house up as it is literally the house of my dreams. I'll share a list of things we do NOT require, either because we already have that space or our family has no need for it:
Any ideas will be welcome- even if you think you're being silly.
Play Room (for kids or adults).
Bedroom (for future kids?).
Lodger's Room (supplementing the mortgage).
Housing a family of Refugees for free (if you're following the latest trend).
Personally, my choice would be a Music Room, where I could keep all my instruments & other musical paraphernalia.
Why don't you wait. You may develop other interest, example: sewing room.
or a room your kids may need later.
it could be a extra T.V. room/guest room until you decide or really need it.
It seems you always need more room.