Could you please explain or give an example of what you mean by journey?
Just to get a grasp of what you have in mind -
Is your 'main' journey the way you progress through the game to reach the end-game or?
Helping for a tough mob, gathering materials ect. feels like natural activities in an MMO, right? I'm not sure if putting up what essentially will seem like a mission/bounty a good thing in all cases.
Up until now, in the MMOs I've played helping for a tough mob or getting help for building a house were all done by the guild or spammed/asked for help in a global chat or my favorite part - setting up a bounty to kill someone in pvp, because someone wanted to get revenge. Setting up a journey(bounty/guild mission(?)) for such activities does seem like a good idea to me, but for activies like gathering materials essentially it sounds like you put up an advert in an auction house/market place for 50 apples for X amount of gold, which defeats the purpose.
If a journey is completed, what will the rewards be? Will the completion give experience or only materialistic rewards like gold/items/mats ect. ? What will the rewards be based on - what the creator of the journey decides or an arbitrary system that decides it based on how long/difficult(based on a very complex formula from locations, mob levels, travel distance, completion time ect.) ? Because this sounds very exploitable and also maybe even impossible, depending on the structure of the open world BUT it does open the room for a competition for who can be the best 'dungeon master' :D Having an 'open' leveling system where the players goes wherever he wants and completes quests made by other players sounds great, but also as I said, very easily exploitable.