I don't know if this has been suggested before (yes, I did search, but couldn't find anything with the broad search terms I had to use), but I think the player ought to be able to bind the mouse4 and mouse5 buttons (the buttons on the sides of some mouses), if their mouse has them. I know most people don't have them on their mouse, but it's absolutely indispensible for me. I like to even the workload on each of my hands when playing, and like to keep most non-movement actions on my right hand - but there's only three buttons on the mouse to bind, and at LEAST 5 actions i would love to use.
Lemme know what you think, would you use this? I think this is a situation where there's no good reason to exclude this, just don't use it if it's there.