Hm...enlightened self interest sounds like it would fit the plot fairly well. I want the story to be introspective for each character in some way- maybe obtaining enlightenment will be their goal. However, naturally, they'll all have their own view on what 'enlightenment' is. (Ascension for the mage, Golconda for the vampire(though they might not even be interested in that >_< ) )
I'm trying to avoid making them work together because they have to (though I know I shouldn't be terribly choosy with such a slim chance of success.) If they can tolerate each other, I think that will be enough to send them through the first story. And after they finish that, I think they'll have a bond strong enough to keep them together.
I considered the idea of a master, some kind of powerful mage or something, summoning them all and offering them tutelage. She'd inform them that if they truly wanted to become powerful, like herself, they'd need to work together and learn from one another. But I'm afraid that would feel constricting for them...do you think its too powerful? Like a plot device almost?