You need approval via state legislation to build anything commercial on park land in New York state. Such approval was given for Shea, and CitiField was built under the same authorization. The Tennis center was the product of separate legislation.
The soccer stadium speculation is based on an interpretation of the original Shea legislation that is, let's say, generous. Paraphrasing very loosely from memory, the original legislation says you can build a stadium and facilities related and necessary to the stadium. It doesn't specifically limit it to one stadium, but it's pretty clearly understood that only one was contemplated. But if you take the partial ambiguity and run with it, you can try to build another stadium.
It could work, but it would also almost inevitably invite litigation that would very likely succeed.
