For the real-time chat function, I had a new idea yesterday: use 3 commands ingame: whisper, chat, and yell. And disable chatting outside the game. That way plans should be made beforehand, and then you'd have to hope everyone does what he has to do; just like it would be in real life. With the 3 different commands, you could also backstab another player by whispering a plan to another, and killing the third one together, should be fun. Chatting is heard in a certain radius around you, and yelling is heard across the dungeon. As a downside, if you yell, sentient monsters would be able to track you down by the sound.
I don't know yet what the coding difficulty would be, but I bet it would be fun to have.