Target of Target
Being able to /assist a groupmate would be nice, though they do that with ctrl-T and then just T to nab the chosen target.
However, the game’s design philosophy involves minimizing UI use. You learn to see which way the opponent is facing, to see them glowing/rearing back/pawing the ground and thus know the big attack’s coming. Cast bars would mean playing the UI, not the world.
It took a while for me to adjust to this after my plethora of WoW mods but now it makes combat feel much more dynamic and fun for me.
and in addition to what Donari said a target of target simply wouldn’t work, because you dont necessarily need to target something to hit it, hell in some cases you can hit something other than you have targeted (if for example you swing your swrod or other melee weapon you hit what is in front of you, even if you ahve target on a person/mob 900 range away