theres severalw ays to do this, the easies iis to set it as @focus , then focus your target at the start of w/e your doing together, will save you time from editing a new name in the macro every time you need it.
that being said this will be ability dependent. example a targets spell, such as blizzard, death and decay, traps, basically anything that gives you that green targeting circle when you cast it, you cannot have that cast at the location of anyone but yourself. example death and decay can be casted @player which will drop it at your feet centered on you and make it function kinda like a pallys consecration but lets day a hunters misdirect can be @target or @focus
some quick examples, ill remove the bits of these popular macros that have no significance to your question.
/cast [@focus] Misdirection
/cast [@“Tank”] Misdirection
“tank” replaced with tanks name and without the quotes. @ is short for target=
mage blizzard (dunny why youde want it but its a valid example) this centers blizzard to the player
/cast [@player] blizzard
/cast [@focus] blizzard
/cast [@“Tank”] blizzard
will not work as its a targets spell (the green targeting circle)
there were 2 or 3 occasions where xpacks and/or game patches made targeting other players/npc/pets with thoes types of spells so they may of worked but blizzard purposely breaks this behavior so even if if ever were to work one day again its almost certain shortly after it would be patched.
so lets look at the macro you want to fuction.
/castsequence [target=player] Plague Strike(Rank 6), Frost Touch(Rank 5), Death and Decay(Rank 4), Pestilence, Plague Strike(Rank 6), Frost Touch(Rank 5), Blood Strike( Rank 6)
because dnd is in this cast sequence, it will lock it up as the game wont allow the cast of dnd at [target=player]
now change it to this:
/castsequence [@player] Plague Strike(Rank 6), Frost Touch(Rank 5), Death and Decay(Rank 4), Pestilence, Plague Strike(Rank 6), Frost Touch(Rank 5), Blood Strike( Rank 6)
BUT it will DnD at YOU, but being melee your likly to be in rang of your target anyway. short of that its @cursor
hope this helps.