Hello,
Since I am a developer I understand the complexities faced by the developer if the User Interface is not properly done, or if it is against the guidelines which either Android/Apple doesn't allow.
I will provide the following:
1. Provide all the designs in Adobe Illustrator format.
2. Provide all the icons and asset files like icons, fonts, etc.
3. Provide 2 design approach, if the first one is not acceptable.
Feel free to contact if you have any queries.