Guides that are at an angle instead of just horizontal and vertical.
Would need 2 ways of making these.
1) enter the angle directly
2) a tool that puts a couple of 'handles' on an existing guide that can be moved to create an angle manually
A tooltip rollover would be nice for the manually angled (& really for all angled) guides that shows the numerical value of the angle so that others can be created to match.
I second that!