Designing graphical users is a combination of different skills. Probably the artistic angle is the most important, but user experience comes close. In Delphi Rapid Application Development paradigm there is hardly the prototyping phase. Your rough designs very quickly turn into production applications.
The key element in visual GUIs are graphics assets and specifically having a really nice set of icons can make a big difference. When playing with a new design for a Windows 10 VCL app, I have come across the most interesting place for free and commercial icons I have ever seen. Icons8.
Icons8 is not just another website where you can download a zip file with a collection of icons, but it is much more then this. There is a client desktop app for Windows and Mac that integrates with icons8 service and makes it super easy to select and adjust the icon that you need. Icons8 business model is very pragmatic and you can use PNG icons up to 100 pixels for free in open source projects, but you can also get paid monthly plan and have access to all icon formats and sizes.
There are certain GUI designs that really intrigues me. Mobile world is taken for granted, but also on Windows 10 there are some really cool designs. Reading "design guidelines" documents is one thing, but using an existing app is something different. A good example of a clear GUI design is "Fresh Paint" app that comes out-of-the-box with Windows 10.
It would be very interesting to design a similar GUI in Delphi VCL or FireMonkey.