Character limits are always tough for an RP. You want everyone to have a chance to play, but it just isn't sustainable to do so. I have a few ideas on a "workaround". If you'd be interested you could always do a part two to this RP. That could give other characters a chance to have an adventure shaped in some ways by the adventure our characters had. 

Alternatively, you could allow people to create NPC characters and crew mates. You'd control them in whatever capacity you saw fit, for now. This way if a person drops out and/or their character dies you can organically add in a new player without it feeling odd. They'd already be, at least fairly, familiar to readers and players. This may be a lot of work on your part though, so these solutions are by no means perfect. 

Also, I'm excited to see the other characters that come from this. I think I'm the only one who's finished a character so far, but I'm not sure. If anyone needs any ideas or wants to discuss pre-existing relationships, let me know.