Please provide an option to manually set tab order over main- and (multiple) sub- and sub-sub-forms.
The method should be as such that a list can be created by adding/deleting/moving up-down that what needs to have a tab-order, independent of the number and/or type of forms present.
The method currently provided (LO 5.2.3 and previous) is absolutely inadequate for purposes of designing (professional) forms.
Example of functional tab order is present in Adobe Acrobat.