Cassie could possibly fit in as the voice of reason when it comes to moral issues faced on the journey.
Ha! I think we're all a bit tired of hearing that... But seriously, good point concerning why she would go along.
And no one stays depressed forever (no, not even Tobias). He'd eventually get over Rachel so why not Melissa? You could justify both being on the fleet in some way and it would only seem natural that they would bond over their losses (I must insist that Melissa loses something other than just Rachel - i.e. her father or somebody).
Judging by the response, this actually might be a feasible spinoff that people would read (grudgingly or not).
But you can't totally alienate fans by marginalizing Cassie and Tobias. There is no way they would sit it out. (I also liked the comments about the tension possibilities between Cassie and Jake - lots to do there)