Try planning a whole situation in your mind, now try directing it.
There we go. Probably.
I believe we need sequential markers, but of multiple groups. ie. [1, 2, 3...], [a, b, c...] and [α, β, γ...]. Very very preferably of different colors for the same group, but different between them, ie. red numbers, yellow letters, blue Greek letters, etc.
We also need clear markers, specially "emergency", big markers. Say a big red arrow pointing down on top of the mob's name, slightly bigger than the rest of the markers.
Then things like shields (which could easily be a signal for tanks), a sword with a big ass red cross on top for the good old "FOR THE LOVE OF GOD DON'T ATTACK THIS" situation, etc.
Do we have Zzz's or a shackled boot (I think we had the boot...) for things like Crowd Control or such? I haven't played in... ages.
Oh darn, and make a kite. With cute little ribbons bellow, please. Preferably red. Yeah, they gotta be red. Red.
You know, those are, for the most part, icons that are easily associated, thus requiring little explanation and easier to remember.
An important note I'd like to add:
There's two small UI element I think would really make this a valuable experience, given our control methods (mostly controllers).
[Optional Multiplicity]
I can't honestly remember if we can mark multiple targets with the same icon-- whether we can or not, that's probably still a problem. It would be fantastic if we could toggle it manually on and off for each icon.
An example of when off (multi) would be useful:
Selecting the "Concentrate on THIS mob!" signal and switching it. Say you are fighting a big fight, everyone concentrated on a specific mob, then suddenly something shows up that you deem more necessary to take down ASAP. Quickly taking the marker off the current target to scroll to the new guy and then marking it can be very cumbersome with a controller, specially if the current target wasn't even on the mark carrier!
Allowing a player to "turn off multiplicity", would mean that marking someone else with that mark will take it off from the original. Goes without explanation how much better that would be on the first situation, right? In big fights, having a slight confusion on the current target can go very awry.
An example of when on (multi) would be useful:
Suppose you are in a fight where crowd control is intensive. Well, teaching everyone different symbols for the same thing is impractical, just use [this] for sleep and give the same symbol on everything you want CC'd.
New mob just showed up?! Use the same symbol you've been using.
Goes without saying, these two situations aren't exclusive.
[Automatic Sequential Rotation]
This could get a bit more complicated to implement than the past suggestion, but I trust it's no rocket science, bear with me here while I explain how this could help "sequential markers" implementation too.
Let's assume we are marking monsters with the "Numbers" sequential, there's two points of discussion here:
a)They should be "interactive" between each other. Let's assume you marked four monsters, 1 through 4. When the mob marked 1 dies, the rest of the numbers should roll over to their appropriate position: Mob marked 2, should now read 1,monster marked 3 should now read 2, and so on.
This should apply even if the number cut was in the middle, for example, we have mobs 1, 2, and 3. Number 2 was just killed, or had the icon manually removed by the player. 1 should still be 1, 3 should now be 2 and 4 should now be 3.
b)Sequential markers could have two "buttons" (or marker icons in the menu), "First" and "Last". This way, we could easily mark using only "Last", as pressing through three monsters would mark them 1, 2 and 3 using only one command/button. If for some reason a new mob appears that should be ahead in the sequence, the "First" button will mark it so (1), rotating the rest. It seems practical to me that the "Last" button be used to "delete" the last number from the list, this way double tapping "Last" on any mob would remove it (as it first would grant it the last number, then delete it).
Unrelated to the rest: A tiny player identifier would also be rather helpful, though not really necessary at all (it could help identify jerkwards that mark when they shouldn't be the ones marking). Like a small "P1" besides the marker for the player holding position 1 in the party or something.
I hate making such detailed suggestions, I know it's probably very infuriating. I'm sorry about that and for the extremely lengthy post. Really gotta wonder if anyone bothers wasting time reading all this crap.