Not super practical in most cases, but I remodeled the bathroom last month and managed to double the closet space. We did a purge of all the clothing that would never be worn again (I got rid of a couple of t-shirts so my wife didn't feel like I wasn't trying).
We're still in remodel mode, but that spare closet is going to turn into built in cabinet/drawer system with space for another gun safe. It has a 6x6 door opening, is 8x8 interior and 30" deep. I'll leave 3 feet of closet rod on one end, put a cabinet under that and the rest will be cabinets with adjustable shelves.
At my previous house, I had two 4x8 hanging ceiling storage racks in the garage. That worked really well.
Jeremy