I'm going to play a human whose martial arts and ki techniques make him superhuman without any necessary augmentations. He is going to be the One Punch Man (not literal One Punch Man but the concept that a normal human can be powerful) of the game.

Also, yeah, I think the first arc being about some travelling Z warrior running into others by happenstance or some unknown fate to be a good idea. It allows each of those players' characters to shine. But you may have to put a player cut-off limit during Arc 1 or we will never see the end of the arc if newbs keep popping in: "Accepting anymooo~ore?" You can expect "drop-outs" since they always happen, so maybe keeping the new and interested players on a waiting list until the arc is completed will help you keep track of prospects.