Well, if want to sort them into terms of Fighter/Mage/Thief, the Engineer is a black mage, medic is a white mage and demoman is kind of a magic knight type thing. Merasmus is clearly boss material, however.
The Engineer is a tad too stationary. Sure, he's got the smarts, but he doesn't seem likely to fill the destructive role beyond defensive measure.
Bare in mind, this is final fantasy logic (and dungeons and dragons if you change names a little). I could see a few of the classes filling "mage" roles in different ways - particularly, the classes with equipment that offers passive bonuses (Demoman, Soldier, Sniper) as opposed to activated attacks. The Medic would definatly make a good magician (especially as he seems rather enthralled by magic while fighting Merasmus).
That being said, it'd be pretty cool if they did release a magician pack of some sort, or maybe a fantasy pack in general? Hmm...
Hmmm indeed. My main reasoning is that he's the squishiest class, and his strategy consists largely of building a destructive caster and diving for cover. That said, it world be kinda awesome if a fantasy pack or wizard-ish class were released. have fun!