This would be nice, especially with a tileset where we don't need to blot out what we're firing at to display things. Simply put, I would dearly love to know exactly what tiles my shotgun/rocketlauncher/bfg/hellwave pack is going to hit before using them. This is already implemented for line of fire for basic shots, I think adding the same functionality to the resulting explosions would make the interface a lot better. Maybe make it an optional thing for ASCII, since there it'll cover up things (unless you can do it with highlighting or something.)
As an additional note, adding the follow through trajectory for the normal line of fire would be nice too, for when you're going to blast though a monster/wall or miss shots.