Two tendencies come to mind:

• (This is more about phonology than morphology.) Tonal languages tend to occur in humid climates, because tones are harder to produce with a dry throat.
• Adult language learners often simplify complex morphology. Therefore, languages that have experienced large population influxes through conquest or migration tend to have simpler morphology. Taking a step further, you might conclude that places that are more isolated would be more likely to maintain morphological complexity.