The venue and time of a planned L8 event is the only legitimate secret I can think of. But as soon as the first L8 portal pops up you are on the map. And the event is no longer a secret. If you are lucky the nearest high level enemy agent is at least 40 minutes away so you can hack to burnout. L8 Farms don't survive.
The concept of L8 farms and how it is done: no secret. People are doing it everywhere, and I get the impression they are doing it bigger and better everywhere else. NYC is in a clam, sort of.
The biggest mistake being made by both teams in NYC is the whole secrecy thing. And so NYC Ingress is nowhere on the global Ingress map. I think both teams should actively blog about their exploits and adventures.
A L8 Farm is not worth global attention. It is routine stuff. But when you create a cluster of 380 L8 Portals, that is worth global attention. And that did not happen in NYC. I think it would be impossible to create more than 100 simultaneous L8 portals by either side in NYC.
Simultaneous L8 farms, consecutive L8 farms: no secret. Neither team has enough active L8 agents to bring them about.
Attack events involving multiple agents to sweep specific sections of the city multiple times on the same day: that was an event idea I had. But I did not feel unique in coming up with it. Once there are more active agents on both sides we will see more such attack events.
Getting together to farm L7 and L8 portals is the most popular group activity. But both groups still leave attacking to solo action items. You go, you attack. Why do you need more than one person? That is the thinking. I don't agree with it. Larger the number of agents involved, more complex the attack event, greater the fun.
The concept of attack events involving many agents: no secret.
What is holding the game back is there are not enough agents in the field. Niantic, I want you to issue 10 more invites to all agents Level 2 and up.
And good sportsmanship is important. The mark of good sportsmanship is that you truly enjoy meeting agents from both sides. You play as hard as you can, but you then truly enjoy meeting the people you play against.