Just curious, of all the background/minor characters that have died throughout the series, which ones have interested you the most and made you think "What if they didn't die?" and perhaps curious what their lives could have been like.
For me, I would have liked to see more about Sei. It was really depressing that she died so early without really getting to have any chance to develop with Kei. While I love Tae, I would have liked to have seen how that relationship would have worked out. Plus, she really did seem like an interesting character, although not really fleshed out enough to know a lot about her.
And more of a slightly mainish-character, I'd like to know more about Nishi's life outside of Gantz. We never really get to see even a glimpse of what he's like out in the real world. Would be interesting to see, specially after his one little mini-speech after the fight with the Tanaka alien.