+1 - better keystone correction.
I'd like to go one further than "draw lines" and be able to "select an edge or line" in the picture.
To expand on this a little..
Many images that we take already have the "horizontal" and "vertical" lines that we want to be straight up and down. Requiring us to draw new lines over those to get the correct angle required just introduces the potential for error because it is often very hard to properly overlay the tool line with what is in the image. (Sometimes the tool line is thicker than the line in the image!)
To take a classic example, picture at the beach with ocean meeting sky line. When I take the image, the camera isn't perfectly horizontal so I need to correct the image. If I can select the boundary of the ocean/sky as being the line then LR should be able to work out what rotation is required to have both endpoints level and also apply distortion correction for any curvature found in the line. If LR was really smart, it could use the information it learns from the distortion correction from that image with other images that I've taken with the same lens, using the same zoom/f-stop settings.
Unfortunately it may be that the number of images with which this works is not sufficiently large to merit this kind of feature being added however it would also work well for architectural photographs where there are lots of straight lines from buildings that often need to be corrected. For sports, I can't see it helping, similarly for a lot of landscape work where straight lines are absent, although maybe it could use things like power poles or fences or maybe even trees (trees might take a lot of extra work.)
In short, often when it comes to horizontal/vertical distortion correction the lines that I want to correct are already in the picture so why do I need to draw new ones?