You kidding?
Tracey's basically a dead character, he has no reason to have him travel. He was really uninteresting in the first place. And why would one of Misty's sisters travel with him? They're basically just there to make Misty feel bad.
Gary's following Oak's footsteps in Kanto. He's busy doing stuff there, he can't be bothered to travel with Ash.
A 5th gen girl is possible, but I'd rather have a 5th Gen male, or both. That'd be cool.
Yeah Make it better
Tracey and Gary are doing scientific investigations in the 5th Gen Region with Daisy who is travelling, (Make one of Misty's Sisters useful) and A 5th Gen Girl who I explained before to even it out
The other is highy impossible but It would be cool if it could work out.