Because Spain was allied to France during the French and Indian War, Britain awards Louisiana to Portugal rather than Spain. How would history be different?
Britain didn't award Louisiana to Spain after the Seven Years' War because it was the possession of the King of France. The eastern portion of Louisiana was given to England as a result of Louis XV's loss in the North American theatre, whereas western Louisiana was ceded to Spain, the partner in the Bourbon Family Pact, in order to keep it from falling to the English. Versailles simply couldn't afford to maintain Louisiana any more and wanted the colony to pass into good hands. The cession made sense, since Louisiana was flanked by a Spanish territory and due to the affinity of blood, governing philosophy, and religion between the Bourbon monarchs.
The Portuguese colonies in America, by contrast, were in the southern hemisphere. A better POD would be the Portuguese settling Louisiana at some point in the 1500s, though it would be a direct violation of the Treaty of Tordesillas. One would have to somehow supersede the Treaty of Tordesillas in order to make this plausible. Unfortunately for the Portuguese, they have no reason to renegotiate their arrangement since, due to the preexisting treaty, they have nothing invested in lands any further than 370 leagues west of the Cape Verde islands. One would have to have a different POD, such as Portuguese discovery of the mouth of the Mississippi early after Columbus, or perhaps a successful settlement being founded there at some time during the Iberian Union.
If the Portuguese were to somehow take possession of the Mississippi River valley, however, their natural growth (if they reach what is/was IOTL known as Upper Louisiana) and their tendency to emigrate in high numbers from Portugal and the Azores could cause them to build a very profitable and powerful territory. It's one of the single most valuable lands in the world, based on arable land alone (not to mention gold, iron ore, copper ore, timber, bauxite, coal, Gulf seafood, an incredible abundance of furs, and a richness of game perhaps unequalled anywhere else).