Lets clear a couple of things up ,JFK will never see 500 flights a day by any carrier ever!..
Here's a press release,it states that DL will tear down Terminal 3 and refurbish Terminal two and add about 16 gates to Terminal 4. I've seen pictures of what it would look like and there's no huge Terminal like AA's plan,they just add to Terminal four and refurbish Terminal two which is the worst Terminal anywhere in the US.
The most optimistic number in the press release is 220 flights by 2014,and that includes regional aircraft.
Don't get too excited about this though,they have yet to submit anything for approval to the Port Authority board of commisioners which is equally controlled by NY and NJ. It took almost 18 months for the commisioners to get the CAL and AA Terminal projects at EWR and JFK approved.
So ground breaking may still be a while for a new DL project at JFK.
The NJ commisioneers have to make sure NJ gets something out of every project list that is presented for approval,the DL facility at JFK cannot be built without NJ commisioners approving it. And they will only do that if there's something of importance to NJ which is also put together with DL's project,like a PATH link to EWR.