Currently, corners can only be rounded at whatever size you set. However, it would be great if you could set different styles of corners such as inverted rounded or chamfer as you can in Illustrator.
The goal would be to create a style like the image below:
As a Designer, I also feel handcuffed in this regard. I worry that the resticted toolset will only produce very similar results. To fix this, I would suggest a feature that lets you define your very own vector-based corner styles.
The ability to round one, two, three or all four corners of a button would be great too. I know you have the ability in SKETCH to set the corner radius individually.
You can do this with shapes. Just put a circle shape at the corners and then subtract from the rectangle. Am I missing something in the feature request?