You could just replace the event with Crota's minions attempting to have vengeance upon those who killed their master. In fact that would probably be a neat twist. Have the event server look if any players have defeated Crota and replace the swords event with a vengeance event.

A colleague of mine loved getting the sword and then going to town on a Devil Walker. Likewise I had one Wolf pack show up in the middle of a public event and it got quite hectic. The combo events would be very fun for all, and often add some interesting interaction in the enemy forces on hand that would change up the zone.

Now THAT is a very cool idea; events that are contextually based on your progress each week. After reset, the "blades of crota" minions can be found invading the patrol zones. But if you complete the raid and defeat crota, the blades could be replaced by another world event like the "Vengeance of Crota" you suggested... similar event, but maybe there are some different configurations of enemies, like a couple wizards and an ogre, or something like that.

Patrol zone is supposed to be the casual part of Destiny, but I do love the idea of the occasional event that makes low-level guardians stop in their tracks and think "... maybe I should go the other way..." until a pack of level 32s come swooping in on their sparrows and save the day. Deej talked about these kind of things happening back before Destiny launched, but in reality it just never happens.

Yesterday, we had a Warsat, Urzok, and a Wolfpack all descend on Skywatch at the same time. it was really, really hard to stay alive, and really, really fun. :) (I felt a little bad for the level 4 guy who looked like he was just trying to collect Spinmetal.)