This is just a suggestion, and you may reply with other ideas. I think it'd be great if there were something like hidden missions/achievements, like the game FEZ. I don't know if you've played it yet, but it's full of hidden stuff, has an own alphabet and numeric system and it's full of secrets.

For example, entering a random site (e.g. the page of the listing of people who are traveling to the same place that you) and find a hidden qr code/link on the bottom of the page and some npc comes up when you head to the link saying he needs help, etc.. Then it starts a mission (maybe extending missions to other countries). What do you think?