Fort Zancudo in GTA V served two purposes: a place for players to find content, and a wanted-level escalator. Leonida's equivalent would be a significant part of the map. We don't know if it has something like that yet.
Florida's real military presence
Florida has several military bases - Eglin, Tyndall, Homestead, MacDill. A Leonida analogue would be believable if Rockstar wanted the kind of gameplay it would allow. Completely speculative.
At the time of writing, the trailers do not depict a positively identified military base.