Yeah, as the subject says, I think this game could really do for a mail system of some kind, just in case you wanted to get in touch with someone who is offline and whatnot. From building a mailbox to simply learning a mail skill, it could be integrated together with the Hearth Fire, Log Cabin, any number of current buildings; or, it could be it's own little construction site. Personally, I think that it should require that you have a Hearth Fire or a Log Cabin in order to build one, but it should still be it's own entity. Or, rather, it has to be built within X amount of squares from the Fire/Cabin.
This also throws in the possibility of a courier system/quests, but that's probably not within the foreseeable future, so, yeah. I'm going to skip that bit and focus on the we really, really friggin' need mail issue.