Customize new Blueprint & Widget dialog

To edit the default new blueprint dialog class you can use the following in DefaultEditor.ini:

[/Script/UnrealEd.UnrealEdOptions]
; Here we add TFCUserWidget class (UTFCUserWidget)
+NewAssetDefaultClasses=(ClassName="/Script/MyModule.TFCUserWidget", AssetClass="/Script/Engine.Blueprint")

more info here

To edit the new widget dialog you can edit FavoriteWidgetParentClasses in the DefaultEditor.ini or in the project settings

[/Script/UMGEditor.UMGEditorProjectSettings]
+FavoriteWidgetParentClasses=/Script/MyModule.TFCUserWidget