Thomas Haugland Rudfoss commented
This would be a great addition to the 1-1 mapping you have today. I've been using ShareGate not only to migrate from an old SP2010 to SPO site, but also to clean up unused content types and combine some fields. If it was possible to use the first "non-empty" field from a set for a destination field, or even set some criteria that would be excellent! We would be able to use ShareGate not only to migrate content, but also to clean it up.