that depends on Frontier. They will continuously update the game and expand upon it. Adding Mapusaurus is not far fetched. But i think in the first case we'll see Acrocanthosaurus as a large pack-hunting carnivore.
I can tell you however that Mapusaurus, despite being in the same tribe as Giganotosaurus, looks very different. Physically its smaller than the Giga. The individual found was estimated to be 10.2m long. Bones were found that indicated a individual that could be around 12m. Check the wikipedia page because it goes extremely well in detail on Mapusaurus's differences to the Giganotosaurus.
I thought it wouldnt be the best addition to the game, but im confident that all the details mentioned, it would be different enough to warrant addition to JWE.