As I recall, the OTL Inca had a policy of only conquering the territories of civilized peoples. However, the Pampas would prove rather suitable for Incan potato and maize agriculture, being one of the most productive regions of the world, and the andean peoples skilled at adapting their crops to differing climes. Would there ever be any reason at all for the Sapa Inca to forcefully move people en mass to the Pampas? Population pressure, disgruntled conquered peoples? The Inca did have a history of mass deportation and resettlement. Would they be able to contend with the Guaraní and Mapuche? How would the Incan population change? Would it increase their power?