Let players send letters in game, it encourages players to stay in game versus on the forums. I am not suggesting replacing one with the other rather allowing an ingame system to supplement the forum one.
Some PCs have made awesome jobs as being IC messengers, actually! I'd rather see PCs play courier than sending letters ICly to be honest! Hiring PCs to run your messages, make sendings, etc are all very cool moves to make.
Keep the jobs in the hands of the PCs.
That being said, some sort of IG "Sealed Letter" item that on use produces the written letter would be neat.
Pointless, since no plotter worth their salt is going to leave a paper trail. But neat. PC's really could carry messages then.
Just use the forum. Such a system is a waste of time otherwise. The only contribution I see it being is the possibility of intercepting such missives- but then, why would you take the risk if you can shoot a PM on the forum?
Not happening, tbh.
This already exists. You can literally use a writing tablet or pay people to wrte sealed notes........Few PCS just bother to use them.