Something that would be a huge leap forward for the scripting language is the ability to perform basic arithmetic and get the values of variables. This would allow numerous new possibilities, such as:
Adding a value to the temperature of every biome in a set
Dividing the humidity of each biome in a set by 2

If you want to do this some other way, that's totally fine, but my idea on what the syntax for this could look like:
biomeSet.set("variable_name", "variable_name" + 0.25)
biomeSet.set("variable_name", "variable_name" / 2.0)
biomeSet.set("variable_name", "variable_name" * "other_variable_name")

As a way to solve this easier you can use eval!
https://stackoverflow.com/questions/2605032/is-there-an-eval-function-in-java