Fan Showcases unlock when you gain a proper amount of fans, so play the career with your band and try to earn as much fans as possible.
You can recognise a Showcase by searching for "Tribute" shows in the career.
When you unlock a Fan Showcase, play it! I was struggling to unlock the last Showcases but when I started playing them, the next ones unlocked quite easily. Also, it helps when you have bought DLC, since you have more songs to form a tribute from.