So explanation of the background about the suggestion:
We’ve had wipes because our tank was desync’ed so we couldn’t tell how much hp the tank had and the tank couldn’t hear us or was ignoring us. In fact, we didn’t know he was still in the dungeon until we were preparing for the boss and suddenly the boss entered aggro mode. Made the entire dungeon a mess until we just gave up because if we let him tank, he’d die and we’d just wipe. If I switched to tank we’d get hit by a desync’ed attack (usually seems to happen when you have two tanks or a dps that pulls the aggro off the desync’ed tank), the boss will start doing a second set of attacks that you can’t see).
Unlike healer where you can tell you are desync’ed, there is very little to let a dps or tank to know that they are desync’ed. Even worse if they can hear you and talk to you, but are desync’ed just enough to matter.
However, if we could send text messages to our party members, it might help (or signboard emotes like: Wait!).
There are also times when someone can’t talk (bug or real life issues) and it might be useful if they could do a quick message like: Wait or Need time.