Initially my thought process was that a human would fill the roll of the Titan, not only as the defenders of the wall but I would imagine humans would be the "last standing" to defend the last city and the final resistance for earth itself in those final hours.
I could, however, imagine a Titan corp made up mostly of humans and exos defending the city and it's inhabitants with their dying breaths.
For hunters I figured that with the fall of the majority of mankind all those exos designed for help would find themselves abandoned as their "owners" went off to fight in the final battles against the darkness. When they never returned the exos turned into refugees & outcasts. This could have put a burden on the remaining surviving cities resources and as supplies, room and power became shorter in supply the exos were eventually "pushed out" to fend for themselves as they didn't really require the necessities their human counterparts required. Harboring no real ill will towards humans exos became scavengers and hunters eeking out an existence in a very hostile world.
Finally I think the awoken just fit that warlock description as they ventured out into space and possibly came across something in the darkness that forever changed them. I see those pre-awoken out on the verge of our inner solar system experimenting with dark elements and something went wrong (or right) that forever changed them.
However, I could see exo titans and human hunters as well in these scenarios. Even a few outlier awoken that have not forsaken their human origins fitting into any of these classes as well.
I didn't think too much about it during the beta as I just wanted to get into it right away, but now, I think I will really put some thought into MY character's personal background/history when creating my class in the final game.
NOTE: passive race stats could throw all this out the window so I kind of don't want to see anything like that in the retail version.
God speed Guardians