It would be great to have the ability, either via list import, or possibly on form submissions, to set data being mapped to certain fields to append to existing data in that particular field, rather than overwrite it.
One example would be for the Description SFDC field, which is a long text field. This field is probably commonly used by sales reps to write in call notes, or used by the marketing team to write in campaign notes of some sort. It would be great to have the option to append notes from other campaigns to this field on a list load, rather than either have to map additional notes to a new field or be forced to overwrite whatever is already in this field.