It would be great to have multiple input types for Flow buttons:
- Text (you already have this)
- Radio buttons
- Checkboxes
- Email
- Date (date picker?)
- Number
- Dropdowns
- Managed metadata (from SharePoint)
- etcetera
That would make it a lot more useful and easy to use for end users! 
We have recently released this feature, read more here: https://flow.microsoft.com/blog/december-2018-updates/