The Fields Groups in any Entity in CDS is really nice feature. One thing missing is that there should be a Field Group Designed for accessing an entity via Excel's CDS add in. Today, If I Open in Excel an entity, columns already added to excel worksheet from that entity are a lot, usually much more than a user needs, and so far, i do not recognise how Excel add in decides which column to add.
If I do it the other way by inserting to a new worksheet the CDS add in, (the method i prefer, so i have the sheets formatted based on my Excel theme rather than default theme), then it is a long way until i add desired columns.
So in Short, i suggest add a New Field Group, to be blank is okay, so developers decide which columns to include by default when Opening an Entity in Excel or Accessing an Entity via Inserting excel CDS Add.