TLDR: The world's your canvas - check this sample app out.
This is great for simple symbology: solid colours, general hashes and styles.
The style you want not on the list? - Fear not! There's a symbol type called PictureFillSymbol.
PictureFillSymbol lets you use an image as the fill type. Here's an example of a cat image inside a polygon.
Apart from cute cats, it's best to use seamless textures and images. This means that if an image/pattern is duplicated many times then you can't see the edges of the texture.
This image is from 100 free Monochrome Geometric Patterns
I created a module a few weeks back called PictureFillSymbolPro. This basically adds a little more customisation options with PictureFillSymbol. This will allow the texture to snap to the world rather than snapping to the view.
Lastly it's worth noting that the outer line/stroke for a polygon can be customised. The outer line is styled using the SimpleLineSymbol.
Here's the list of line styles too -
*You can change the colour of these different styles using css colour string, hex or rgba.
Here's a simple example that uses all of the mentioned above.
The legend will keep to the same style as the features by default, including any animated images you use.
You can pretty much do anything you'd like. There's some slight workarounds and tricks to getting better performance as well as better symbology. But generally it's rather simple and easy to use.