I think the CoF shouldn't be exclusive to a specific world event. Just have every world event have a change of spawning the CoF (for instance every world event has a 25% of having a CoF).
One of the problems is people constantly hopping servers looking for that one single world event they need. This is problematic because other servers keep having 1 or more slots that are just taking up by people imidiately leaving because there isn't a FoF (this on top of the people doing hourglass dives). On the other side the servers with a FoF are being swamped by very highly skilled players, making it very frustrating for the casual playerbase to go after that world event and try and find a CoF.
And the other world events are still not interesting enough, especially considering the risk/reward structure.
By having the CoF have a chance to spawn in every world event, every world event becomes interesting and server hopping just to get the correct world event would drastically reduce.