I think that you want major game changing events;
a. A portal opens from another dimension and an army of demons/undead/spiders pour out to conquer the surrounding lands. They will continue to do so until the portal is closed by exploring the ruins that is the source of these units. This army is a threat to all sovereigns.
b. An great army lands in ships from a far off land. They proceed to attack, conquer and pillage cities as they find them. Destorying cities, roads and mines. This army is a threat to all sovereigns. Think - Mongol Hordes
c. A great darkness has been released on the lands. Until the ruins that are its source are explored, food production for all all lands is reduced by half and excess population experiences starvation.
d. Bubonic plague has been unleashed affecting all cities linked by trade routes. Reduces population each turn until sanitation code is researched or the plague dies out. Armies and champions visiting a plague city are infected and suffer losses.
e. The dragons are united by one ancient dragon. The dragons demand tribute from all sovereigns. Sovereigns that decline to make a tribute will see their cities attacked and laid waste to by a dragon attack that destroys many buildings in the target city. These demands for tribute will continue until the dragon lord is slain - this ancient and powerful dragon resides in the ruins that must be explored - who is feared by all dragons. Tribute may be gold, magical items or the sacrifice of champions.