Can I develop my own custom elements?
Yes. If you're a developer you can create your own custom elements. These will appear within the 'Custom element' category in the Sidebar browser.
If you require a specific Element and it's not available in the Sidebar browser you can develop your own ones. These will be made available within the 'Custom element' category within the Sidebar browser and can be used on the Layout canvas.
Here's a developer guide to creating Custom elements.