I believe a portion of the Shipyards' acreage is underwater and the JEA property is about as far away from downtown as you can get, yet still be considered as being in downtown. While Springfield's chain of parks may only be 37 acres, they are the closest thing we have to the previously mentioned Boston Commons.

The Commons covers 50 acres but most of that land is not flex space for temporary events. There is a major street that bisects the space, large ponds, baseball fields and heavily wooded areas. Nevertheless, it has accomodated +100,000 crowds for special events in the past. The key thing in successful urban park design is that the space be designed for a diverse amount of everyday use, even if the goal is creating land for mega temporary events. If flex space for temporary mega events is desired, look no further than the Sports District's parking lots and Metropolitan Park.
Jacksonville Sports District
You'll find acres and acres of asphalt that sit empty for most of the year. Before spending millions of taxpayer dollars purchasing expensive waterfront brownfield sites, maybe we can discover a way to turn the entire Sports District into a mini version of Chicago's +300 acre Grant Park?
Soldier Field is served by underground parking and mass transit.
Our Sports District could benefit from a skyway extension, streetcar line down Bay & Philip Randolph or both. Having viable mass transit options would reduce the need for surface parking. Tailgating could still be accomodated in permanent multi-use green space areas.
Flex Space
surface lots could be redeveloped for "flex use". In Grant Park, these baseball fields join to form space for larger temporary events.
Just because there are existing streets does not mean that smaller parcels can not play a role in larger temporary eventsReusing our already owned poorly utilized surface lots in the Sports District and Metropolitan Park allows us public riverfront access without purchasing expensive private property. Jacksonville's true Central Park could also be tied in via a greenway along Hogans Creek, which empties into the river near the coffee plant. With coordinated long term planning, paddleboats, gondolas and canoeing opportunities could be made available.