To predict the outcome of Antonio de San Buenaventura y Olivares's decision to start building a mission in 1709, one must consider the historical context of interactions between Spanish missionaries and indigenous groups, including the Tejas.
Given that the Tejas were known to have been resistant to Spanish colonization efforts in the past, it is likely that resistance would be encountered. Furthermore, the success of missions often depended on various factors, including support from local populations, resources, and the ability to establish lasting relationships.
Considering these factors, the most reasonable prediction would be: He would have met with resistance from the Tejas, and the mission would have failed.