The Button data type is currently on our roadmap.
A dropdown (Select or Multiselect) with Country flags can be created by using flag emojis icons today. We are also imminently releasing the ability to define Select and Multiselect options from a range or named range, allowing easy re-use of values in dropdowns from any range in any worksheet within a workbook (similar to how traditional spreadsheets allow data validation).
Regarding cascading drop-downs, do you have an example you can point us to? Curious to learn more about what you have in mind.