I think it would also help to have more commands with inbuilt markers.
Sure, you can theoretically make them yourself but that's something more advanced players tend to do. Casuals or people getting into PvP probably don't know much about common marker customs (e.g., the marker cooldown ending in a circle or the paladin guard marker convention). They also might not know much about creating (more complex) macros to begin with (countdowns or macros that disappear like the PLD guard marker are tricky if you don't know how to write them).
They could also make communication more precise because right now markers are a bit of a "multi-purpose" tool. Ignore-marking someone is probably intuitive at high levels (you know the difference in reason when a SMN is ignore-marked or a tank) but at lower levels it might not be as clear.
The same goes for bind on enemies. Does it mean actually binding/heavy-ing them / hindering them from moving? Does it mean using silence/stun to interrupt an attack? (Does it mean something completely else I'm not aware of? lol) Something that might be common sense for high-level players but not as intuitive for those not as experienced.
So if you could make commands that distinguish between "Ignore this target, attacking them is a waste of time/resources" (e.g., tanks) and "ignore this target right now because they are very dangerous at the moment" or a general "don't ignore but be extra careful with player X" (if someone's very good or on a generally strong job) that would help a great deal (those commands are not word-for-word suggestions btw). It would teach lower-level players awareness and strategies organically and in the moment.
You can also learn this from guides and communities but not everyone seeks out the former or engages with the latter. Like this you could correct mistakes or communicate better gameplay options much more efficiently.
Also, specific ground markers that come with chat commands would be helpful, e.g., "gather up here", "ambush from point X", "enemies incoming from Y" etc.
Another command that comes to mind is "Follow me." Right now we only have indirect ways of saying that such as "I'm going ahead" (please follow me, wink wink, beg, implore, where is everybody?).
(Edit: I just saw that Allicia suggested the same thing. Sorry, I didn't see it earlier.)
"I need healing. I need healing. I need healing."
No but in all seriousness, something like a "help me/assist me/cover me" would be useful, esp. if you are targeted by several players.