How creative can we be? One of the great things about Evangelion was how it turned from sci-fi to almost spiritual fiction (at least how I saw it), so there would obviously be a lot of hidden aspects of our characters to develop later on... That being said, are they "just" Eva pilots? Can we have funky-yet-plausible prototypes? Can we go deep in our backgrounds and set up stuff for later on? 

I guess it depends how linear you want to go with the series' storyline.