Any pattern can be made in King size. You can either enlarge the blocks or increase the number of blocks.
My personal favorites would be a large medallion in the center with rows of smaller blocks around it. I've made a large block something like a carpenter's wheel and filled it with a large applique meaningful to the recipient. I've made a lone star. A mariner's compass would be nice.
You'll want the center block to be somewhere between 40 and 60" wide. This way it will fit nicely on a queen sized bed if they eventually downsize. And it will still look good on a king sized bed.
Other possibilities include bargellos and similar types of quilts including around the world.