Ability to add field to SharePoint
Within the old designer, you could add a field to SharePoint - and then use within the form. The new responsive designer doesn't have this.
Workaround is to save/close - and go to the List Settings - add the field, then go back to edit the form. Would be nice to add a field "in the designer" - like with the older/previous forms designer.

Thanks for your suggestion. I will update the ticket if we have anything to share on this idea.
Cheers,
Euan
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Marcel Gilbert commented
This was a nice feature and should be added back.
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Dale commented
Add "Create Column" button back to the Forms designer ribbon. For some reason it is gone, without any explanation. It is still listed on the Nintex Forms for Office 365 help page at https://help.nintex.com/en-US/Office365/Forms/ResponsiveClassicForms/VisualReference/Ribbon.htm