Suggested Additions:
1. Help <t> (for coordinating if you see someone getting ganked in the background)
2. Help me! (If you're the one getting ganked while everyone is chasing the enemy)
3. Conserve LB (For possible burst potential)
4. Use/Using LB (instead of just LB Ready allowing for coordination of targets)
5. Wait! (To try and catch people from running in since fall back and regroup is only for specific instances imo)
6. Enemy Behind us/Pinch/etc (for letting people know someone's coming from a side/behind you)
7. Incoming Field Effect! (For when aoe or bombs are coming up and some people are in the fray, Danger Bongos for PVP)
8. Use Guard/Guarding! (For flagging when you're using your defensive skill, this may be more niche but at least it tells someone that you've used it/are using it)
9. Don't attack <t>! (for moving people off of Tank or healer if they're getting killed trying to go after them. May not be needed with focusing on/attacking <t> )
10. I'm using Tilt Controls! (Kidding)
Essentially all the straightforward stuff that ANYONE would be able to call out if they were able to group up and/or utilize voice chat.
Can you use countdown in pvp? If so I'd love to build something like "LB <t>" then go into countdown. At least then you could herd the team a bit.
If even one person listens to you in the match you'd have a shot at organizing all these randos to possibly making a comeback/winning.