User Setting - Dark Mode
Allow the user to choose between a light and dark theme. Since basically all of the the office Suite offer this choice I think it is only logical to expand this behaviour to the Dynamics 365 product suite.

Tenant based theming
Allow admins to select some theming options for the tenant like a different accent color instead of the default teal one. Maybe the roundness of borders inside the application as well as placing a logo or company themed component on the start page.