If they ever considered moving the stadium, I would propose the likely site would be on land owned by the Davis family south of JTB and north of the county line/Nocatee, They still own thousands of undeveloped acres there and it would be centrally located to the higher income areas of the Beaches, Southside and St. Johns County with interstate-type access from JTB and maybe some new roads connecting from I-295 or the Nocatee Parkway.
Not encouraging the urban sprawl that would bring above and beyond what is already going on around there but I don't think there is a better spot after Downtown. And Khan and the Davis's could combine their billions to build another new "city" around such a stadium. The incentive for them to make the land available for free or a great deal is the roads, cache and traffic that would enhance the value of the rest of their property. Not unlike their donation of 400 acres to Mayo, the Fletcher's donation to the PGA or the Skinner's donating land for UNF.