I swear I thought of this before! I also have a good idea for the mechanics. It starts out very similar to the Rikti/Zombie events in that Arachnos troops spawn at players' locations and when either x number of Arachnos have been defeat or x time has passed, one of the Arachnos patrons spawns (Lord Recluse, Ghost Widow, Scirocco, Black Scorpion or Captain Mako) If the Patron is defeated, the invasion ends.
Villains should also get a similar event featuring Longbow as the invaders and the Signature Heroes (Statesman, Sister Psyche, Synapse, Positron or Manticore).
to 
TO THE END!Villains are those who dedicate their lives to causing mayhem. Villians are people from the planet Villia!
Redacted: Wrong forum!
-Rachel-