One coin I want, but can't quite afford just yet: A 2016-W Standing Liberty gold quarter. (After that, a Walking Liberty gold half dollar. And then a 1 oz gold buffalo.)
My current object of desire is actually a bill. I've always really liked the $10 Gold Certificates from 1907 and 1922, and though I could afford one, I can't bring myself to spend that much for a bill. So I may just enjoy them vicariously. But who knows, I may cave someday.