How about random attacks from the old db generals or notum/fiery/diamondine... not one at a time, all of them at ONCE!!!
Early attack waves would consist on their adds and then their placeholders... and finally then they would come.
Or even... a swarm of cultists invading borealis followed by faithfulls, followed by acolithes, followed by windcallers, folowed by.... you are getting the gist of the things but the final thing would be Hezak, a High LvL version of Khalum and Inobak rampaging.
Or... a mantis invasion, following the same pattern, small mantises... then the breeder sized ones and finally a gigasized Queen is carried into the city by her caretakers.
Or even mercs... All of then rampaging trough Borealis/Old Athens or around Rome Red. Just toss a high reflect or the biggest HP bar AO can have on those big bosses and there ya go fun random raids with no loot involved or if it is less than 50% chance on what they'd regularly drop.