I could have sworn there was a feature request, or at least a small discussion, of this concept before, but alas, I cannot find it. The concept is straightforward and simple, yet highly useful. With a projectile attack, attacks actually are aimed at a location (namely where the unit is standing). So if the unit moves, they can avoid the attack. Thus, the concept being attacking a location is already in the engine.
What I want to see is a separate command which would use the same parameters as a regular attack command (in fact, almost exactly the same command overall), but instead of supplying a unit to attack, you supply a location (if you select a unit though, it would attack the location the unit was in when the command was issued). Thus, the attack is aimed at a specific cell on the map instead of a unit's location. The use for this is apparent with "superweapons", such as UNATF's nuke attack, which deals damage in a large area, and is meant to be aimed at the foe's base, not any specific unit. In fact, as it stands, you have to send a scout into the foe's base in order to "see" a unit to attack the base. With an attack-location command, you'd be able to just point the nuke at the location of the player's base as you rightfully should be able to.
It would also be useful with other large splash attacks, allowing you to aim in front of a moving foe, minimizing misses (for an advanced player).