This is something I've thought about for a while, but I think the addition of a proper messaging system through the in game world would be a nice addition. What I mean by this, is using the actual in game letter system as it's intended, with the help of NPCs, not just PCs. For those that don't know to what I refer, I mean that when you write on a piece of paper, then seal it using a signet ring, it becomes a proper letter than one can open, and it will then say 'This letter's seal has been broken'. I think there could be an interesting way to get NPCs involved in this. Introducing, the Ravenloft Postal Service (RPS).
By allowing certain NPCs an option to 'deliver' letters, this operating as, a player approaches an NPC with an already sealed letter, types out the desired PC's exact name or a place, and 'sends' them a letter by paying a set fee or a fee based on the letter's characters. The 'sending' process I would suggest operates one of two ways,
1. The NPC sends this letter to a 'pool' of letters held by certain NPCs, anyone can pick up their letters from any one of these NPCs. These NPCs would be managers of inns, banks, and certain government officials for factions. Guards of certain areas have access to certain pools, if letters are sent to a place.
2. The letter goes directly to the inventory of the desired player after a set period of time, not /actual/ days, but maybe an hour or more.
The reason I suggest this is not as a, 'THIS SHOULD BE ADDED IN /NOW/', but more as a, 'Hey, here's an idea,'. I know barely anything about NWN scripts, I've looked at it and I mainly just go 'Ah yes... there is code here.', so take this with a grain of salt, I'm not sure if it'd even be possible. I just notice that, while sending letters over the forums is quick and easy, and in my opinion a great way of doing it, most people still just tend to either put in tells or a discord message saying, "Hey, you got a letter that basically explains X,Y and Z.", which I'm not saying there's anything wrong with that, but INSTANT communication like that is simply not realistic, even if you have some PCs that RP handing off a letter to their familiar which I think is still more realistic than just meditating for a moment as you type a tell.
Not to say that I think it should be breaking the rules to send tells or anything. (Though it is breaking the rules to discuss IC conflicts as they are developing but meh.), but I think it would incentivize more RP through an in game letter system. I've asked a few PCs to deliver letters for me before, and I've seen others do it as well, it leads to great RP. Especially if it's a letter that guards shouldn't read and your messenger gets arrested. But now imagine that guards could potentially just go to your mail box as well, (with certain permissions) and be like "HOLY COW. This guy was in X cult and was summoning fish people to attack Vallaki! Very evil guy!" [In Balok]. Or even better, someone witnesses you getting a letter, then they try to ask you what it was about etc. I think it'd be a good RP thing!
I think also it'd be neat if certain factions could get their letters quicker and for no fee, like guards who need messages quickly, or the Red Vardo because... they're the Red Vardo.
Better yet, as I was typing this I also had the thought of actual DELIVERIES, because PCs also do that sometimes. Hire an NPC to SEND ITEMS to other players, granted that would be likely more difficult to add in, but no less interesting! Same process and all!