Team ,
I am looking for a feature to pre-define custom fields and have option to choose from the list when needed. This is a very helpful feature.
Could you provide more info ? Examples, use cases ? 
Sure. Lets say for example for a website , i have an username(which is not email address ) .So while saving the details on BitWarden, on the login field, i give username. But to remember for which email address it has created with, i am adding a custom field as Email.
so for this example, if i define Email as a custom filed, and it should appear on all LOGINS while adding as a custom field.
Hope its clear.