Firky wrote...
1. Why are (according to wiki and codex) desire demons "ambitious" do you reckon?
I agree with Corker on this one; it feels meta.
If you wanted an in character reason though, maybe it's simply due to their supposed intelligence in the hierarchy of demons. Rage and Hunger are kinda primal emotions driven by instinct rather than intellect. Sloth...that's more a trap or lure than a drive, but Torpor in DA2 showed sloth demons
can have ambition. Pride can be taken as practically synonymous with ambition.
If you take Desire as Covetousness (Want without Need), there has to be a reason behind it that transcends mere instinct. There has to be an element of intellect,
why the object of desire is covetted, and an ambition to possess it.
Personally, I think they're just labelled as ambitious because they're more active in seeking out mages than the lower tier demons, and more common (apparently) than pride demons. It's the Codex author's perspective, and not necessarily accurate.
2. Beyond feeding on your desire, what could a desire demon possibly want itsself?
There's the Kitty case Corker cited, from Stone Prisoner.
Connor's demon wanted to possess Connor (she specifically wants to keep the boy and bargains for him).
The Broken Circle demon claims to want to experience life through mortal eyes, and that case is unusual because she does not appear to be directly possessing anyone. According to lore (IIRC), demons who leave the Fade but don't get a host go nuts after a while.
Feynriel's demon(s) in DA2 wants Feynriel as a host due to him being particularly powerful, powerfl enough to intimidate other demons, shape the Fade and influence the world.
Codex lore states that all demons want what mortals have and that they're jealous of 'life'. that they crave it. Justice in Awakening appears to corroborate this by adding that spirits, in contrast to demons, are meant to have no desire for such things.
There are a few Codex entries about what demons go on to do when they've possessed someone. Take the Baroness in Awakening, or this guy from DA: Legends:
The former Viscount Khedra spent most of his life as a venerated ruler, celebrated for his wisdom. Late in life, he started to change, beginning with the construction of a great coliseum where nobles' chosen champions competed for pride and glory. What began as games of entertainment quickly devolved into great spectacles of carnage punctuated by outlandish pageantry. The lesser nobles of Kaiten no longer settled their disputes in court, instead grievances big and small were decided by sword for the pleasure of an increasingly bloodthirsty populace.
The Viscount's nephew, Ravi, and his allies discovered the source of Khedra's odd behavior: Khedra was possessed by a powerful Pride demon and was now an abomination.
Perhaps a desire demon would do something on a thematically appropriate level, if it got into a position to rule an entire city (debauchery, excess etc). Perhaps desire demons want different things...
3. Within the structure of Origins-style player dialogue, how could the demon extract some of what the player desires?
Previous dialogue with other characters that ask specific questions of the PC. You could code the dialogue to add 'points' to specific categories whenever a question is answered to determine what the PC wants most, so when the demons arrives it can automagically know what the PC desires by taking the highest score. Not perfect, but an option.

You could also attempt to base what they want upon their Origin, which is what I wished the Lost in Dreams quest did right at the start. (Weisshaupt, Mr Sloth Demon? [/i]Really?[/i]) It runs the risk of being dead wrong though. One Cousland might be ecstatic at seeing Mum alive and well, another might go "WTFMate?"
Or, I guess, how can the player be set up to really want something?
Heh, you made me remember a short video from a while back where a master of mental manipulation talks to a guy for about five - ten minutes and makes him believe he doesn't want a new bike, he really wants a red car. It was fascinating. All he does is talk, make a bit of friendly body contact, and then at the end the 'victim', when asks what he wants, says he wants a red car with the kind of inflection that suggests he wanted that all along and hasn't had his mind changed. I wish I still had the link.
For the game though, if it's a specific thing we're talking about, you might have to set up a specific situation the thing would be useful for. For instance, curing Arl Eamon. Connor is tempted with a 'cure', and it works. The lonely daughter in Stone Prisoner is tempted by Kitty with attention from a loved one. Sebastian's scheming aunt(?) is tempted with political power.
If a mod was set up along a specific theme that built up a certain desire, promising to answer this 'want' could be quite effective. If it's out of the blue, it's a bit harder as desires can be a lot more varied, which is probably why demons in the games promise the PC mere material and stat-based rewards, nothing that's really
heartfelt.
(On 3 - it is a murder mystery, so the player "should" want to solve it, and they should want to solve it for the benefit of saving a friend, ultimately. But, with players being diverse as they are, how could these goals be well illustrated, I wonder.)
The stakes would have to be high, I think, if you wanted to suck in the highest number of players. Why would the PC want to solve this
more than riches or power? Is the friend facing death for his supposed crime? Torture? Many people will respond if they know someone is in pain and they can end the suffering...others will respond if rescuing the friend will lead to a reward.
In DA2 (Night Terrors) and DA:O (Lost in Dreams) both, the demons snare Feynriel and the Warden's companions with visible illusions and pretend futures, so maybe the PC could be manipulated with something similar. The demon need not be
visible when providing the temptation, at least not initially. You could play out a fake rescue scene, saving the friend from a terrible fate to prey upon those who desire to 'save', and having great reward + praise to prey upon those who desire recognition and reward.