What do you want to do?
Have a pattern fill bucket along with the solid and gradient fill options.

How would it work?
Pretty much like the photoshop pattern fill, it would use primary and secondary selected colours to create the effect. For starters, to avoid having to save custom patterns, you could start with the 4 or so patterns that are commonly recognised as the generic "gradient" steps to dither from one colour to another. I have attached an image showing the different patterns below.