This would be 2-3 planets where evidence of intelligent life exist but they failed at something causing their extinction.
I'm currently floating the idea that Rnin-on, Lith-on, and Filtya-on are examples, perhaps even one society that made it to the galactic age, but then died another way.
I'm also interested in placing Fallen Peoples on some Tutinari colony worlds and one on a barren world where a species seemed to live in vacuum.