This command would bring up a small targeting circle similar to a placed AoE effect. Upon clicking a location, it would command your pet to path to the location you selected and stay there until commanded otherwise.

I don't know about all combat pets, but I'm guessing that most have at least 1 open slot in the petbar for it to be put.

Some props for Trion, I noticed that pet abilities can be dragged and dropped into any action bar, so even if there weren't room in the petbar, you could still use it by jamming it somewhere in your bars.

Blizzard added just such a thing to WoW after 5 years and 2 expansions.
Here's to giving them the what for, and adding said command in >5 months from release.