I have a scenario where a sheet supports a very large range of columns, but in reality only a subset of them are included in each import.

The current options allow me to ignore unmapped columns, but the sheet still becomes really wide if you have many.

I envision an option to configure columns as not only "optional" but also "hidden by default". If the user maps a column in their file to that field, or if it is auto-mapped, it appears. Otherwise, it doesn't show in the validation UI.