Alvar Nunez found himself deep in indian territory in 1528 after the rest of his expedition died off. He became a traveling healer on his journey back to Mexico. It made me think, what if he decided to settle down among the indians? But also what if he discovered or learned back in europe innoculation? He was greatly honored among the indians as a healer and good man, could he become an amazing historical leader?
Yes, he was indeed. However, he was just a man, and, though he could have united the tribes of Texas and Northern Mexico, I doubt he would be able to give them much technology. During his journeys, he adopted Indian ways in order to survive, and not otherwise.
I think he could have been more influential if he had become governor of Northern Mexico after his return or something of the sort, as he could have established much better relations with Indians.
Unfortunately the authorities didn't wanted him there, so they sent him to De la Plata Basin (which was like sending hiim to the other side of the world). He still managed to accomplish great deeds there: he crossed the jungle from Santa Catarina to Asunción, allied with Guarany chiefs on more or less equal terms and discovered Iguazu Falls. However, he managed to indispose with the authorities and some colonists for his views about Amerindians, and was arrested and deported to Spain. His Amerindian friends accompanied in canoes all the way down along the Paraná river. He could have been murdered during the return trip if a storm hadn't frightened the conspirators who thought that God would punish them if they did what they were planning to do. He was put into trial. Despite false testimonies, the authorities eventually realized he was right, but never sent him back to Paraguay.
If there had been a rebellion led by colonists loyal to him and by Guarany Indians, he might have become the ruler of Paraguay, and could have establish an independent republic in which Amerindians and Spanish would be on equal terms. The culture formed would be even more intersting than present-day Paraguay's, and, given its geographical isolation, it might have resisted the Spanish for quite some time (enough to get Portuguese or other country's help). The only way to attack them would be through river De la Plata-Paraná-Paraguay, and, since Buenos Aires had been burn and was depopulated back them, the Spanish would have to mount and expedition in Europe and wouldn't be able to get supplies till they get to paraguay (which would make the expedition logistically impossible).