With the 2019 release wave 2 plan, finally, we can customize the opportunity close dialog box (Unified Interface only)
Enable it from the Sales tab of System Settings dialog box.
The default out of the box Close Opportunity Dialog Box à
Now to add or remove fields to the dialog box, we need to customize the Quick Create form of Close Opportunity entity.
Change the ordering of the fields, added a custom field, removed Competitor and Description dialog box.
Updated Close Opportunity Dialog Box
We can use client-side validation on it
We can specify business rules
We can also customize the Main (Information) form.
This form can be used to check the details of closed opportunities.
Entity Relationships cannot be customized.
Great to see the feature available as it was one of the most requested features in the product.
Hope it helps..