So, this suggestion originally started as a “Cover tool” that would determine how much cover token A had from token B and vice versa, but that seemed a bit too 5e-specific of a tool to be broadly useful, so I simplified it to this: A tool availible to both DMs and Players that would draw a temporary line between each of the 4 corners of a given pair of tokens or tiles and their centers, and displaying how far apart their centers and closest pair of corners are. This can be used not only to resolve cover in D&D and similar systems, but also as a rangefinder for ranged effects and even a crude cone tool. using this tool with Shift would allow you to select a point for one of the two sides of the Line-of-sight tool, both for tighter cones and line-of sight on more specific map locations.
The measure tool has worked well enough for line of sight in my games. The players draw from their token to an enemy token and see if there are obstacles.