Focus on the GUILD MISSIONS, as for now.
I kinda agree with shiroun's idea on the fortress. Actually, just go with any big mission that require several players. We can use that mission as the common-ground for all in-game characters to "coincidently" meet and get to know everyone.
Then after successfully completing it, Yggdrassil's team will suggest forming a clan, leading to everyone joining up. Sounds doable.
And let me in to the clan, too!! Any name is fine with me, but if I were to suggest; how about Clan "Sword for Hire" (SFH) ? LOL
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*Edit:
@Injury: I was thinking, maybe you could add in "Away-Quest" missions, or whatever they are called. It is those quests that require the player to send out one of their NPC (or partner character) to complete it.
The stats of that character will predetermine the outcome of the quest, but ONLY you (the gm) will know the actual criteria for clearing it. Quest like this can take up for one whole mission-time, or end after x number of days (real-time). The sent character will be freed after duration expired.
Example: Simple fetch quest that will need at least 5 in Speed to complete. I join it using Lunette, but as her Spd is inadequate, the mission will end in a failure. Of course, the result will only be announced after said-duration is over.
This could be useful for allocating npcs to do something else, while the main characters go on a combined clan quest. Things can get really messy if you allow the whole clan and their partners on one single mission (think 6 members + partners : 12 characters on field). This way, we can just use our main charas, and let our partners run amok on fetch quest. Just put a good reward (both for fail, or complete) for those simple quests, and we are all set~
*Argh, long suggestion. Sorry about that.