Great idea (he says shuddering still at swamps! :-) ) So like a random encounter after defeating highest boss a "War party Roaming event" that could be prompted by "Odin challenges the Warrior!, A War Party is North of your location..(and you are invited! :-) )" Seek and destroy ... So the first could be 2 random bosses.if you defeat them.. later at random location again it could be 3 or 4 bosses and so on .. Im already scared of my own suggestion) Drops could be 5 or 10 minutes of Ore and or animals thru Portal time..