Target markers over NPC targets (and only NPC targets) would be a perfect addition to this game's UI. Having it centered in the reticle so that it always shows up front no matter what angle you look at it would be the ideal location for it too.
Other MMO's use symbols only, but this game, I think, should have a text box attached to every symbol for basic notes, i.e., "stop DPS at 10%", "ignore this target", etc. Limit it to 32 characters though.
One last thought is that this was mentioned in the forums back in 2010 and it got little feed back - why??? This game is perfect for a UI function like this!
I couldn't find anything cannon related for marking a target, per se, but here is the greek alphabet: