We’ve just launched a new crowd funding campaign to implement a drill-down (cascading) field support within QGIS forms. Full details are available on the campaign page.
This is a really exciting new feature which would help add greater flexibility and power to QGIS feature forms! To make it possible we need 3500€ pledged before 11 May 2018. You can help make this a reality by supporting the campaign or by sharing the page and increasing exposure to the campaign. Updates to follow!