There are only unique lvl 5 upgrades available for cedtain factions. Most of the times it's a yellow city centre building like unique lvl 5 theater for Syracuse or library of alexandria as lvl 5 library for egypt. Other common type is lvl 5 temple. Also, all hellenic factions get one unique lvl4 upgrade to minor town city centre healer upgrade. Some other unique buildings are lvl 5 ports for athens and massala as well as lvl 5 barracks for sparta. If you spot a settlement with those uniqie buildings its recommended to not raze most of the time as you cant get thise buildings yourself. Gotta be flexible.

Cheers!