i always thought these were cool, some ideas that hopefully may help:
-classic warfare idea: a way to get it to be a little more organized(and lore compliant) is to have the mobs sorted into 'groups': archers in the back, sword and axe wielders in the middle, heavy armored ones closer to the front, and one horse mounted void knight leading them all in.... have battalions of about 10 in each group, seeing how there can be anywhere between 15 to 30 people on one mob, it might make it easier for players to fight the void knights if they have designated groups, and for each battalion wiped out, one to two(depending on player population) bosses spawn in...
-player population and lag idea: having the players organized into different groups may ease the population overloads... have the event consist of two attacks, four waves each...
group 1: attack one, wave 1 and 3
group 2: attack one, wave 2 and 4
group 3: attack two, wave 1 and 3
group 4: attack two, wave 2 and 4
each group of players can contain up to 15 to 25 people
keeping 3 out of 4 groups in regalia they can RP and focus on preparing and waiting for their turn to fight may help lag spikes that result from having 40 players in the same chunk...
-staff?: these ideas sound ehh...okay on paper, but i honestly don't know how the staff would handle it... these are only ideas, hopefully it will help the staff members???