I suppose you could set up something on a round-robin.

Have the first press send (Tab) to all characters.
Then the second press does not send (Tab) to the initial toon, but does to all of the others.
The the third press does not send (Tab) to the first two toons, but does to all of the others.
Etc.

Not sure if /TargetEnemyPlayer can be pressed multiple times, and will cycle through the targets or not. If that is the case, that will definitely be better then straight Tab which will get pets, totems, treants and such.