While I like the SHIFT-and-drag feature for the brush tool to paint straight lines, there are cases, when the “Photoshop-way” is better - especially when I want to vary the brush stroke through pen pressure while painting.
My suggestion would be the following. When you press SHIFT before you start the brush stroke, it should be as it is now - basically switching to the Line tool. But when you press SHIFT after you started the stroke (now nothing happens) it should snap to a straight line (horizontal and vertical should be enough). Not a major deal breaker, but a nice addition to the functionality of the brush tool.