This is a very interesting question. What would Rachel do if she had lived? I hated KA's conclusion to Rachel's story. I hated even more her explanation. That Rachel was deteriorating, losing herself to the war. And she'd never be able to recuperate had she lived on. So death was her only option. Nice way to keep your characters inspirational. Had she no hope for Rachel at all?
I agree someone had to die, but I really would have liked it if it had been Jake. Everyone would be sad, but at least they'd be able to move on with their lives. Cassie would definitely get over it. Eventually. Well, not entirely, but she'd be able to move on.
Anyway, maybe Rachel wouldnt be as bloodthirsty and wild as KA so heavily implied, after the war. just because she's an adrenaline junkie doesnt mean she cant learn to assimilate into normal society. She'd be like a soldier returning home from the war in Iraq. Transition might be difficult, but she's a freakin Animorph? It's not like she cant morph anymore. And that alone might be the escape she would need, the adrenaline rush, while living a normal life with Tobias. Hell, she might even do the fashion design thing on the side.
After she died, I was hoping the Ellimist would come to her like he did and enlist her as one of his warriors, protecting the galaxy in the intergalactic chess game. Like the Crayak had the Drode, Howlers, and other agents. I know she would have excelled at that. And more importantly, been happy doing so.
The Rachel who narrated in the beginning of The Beginning was perfectly sane, smart, and looking forward to the war ending. She even goes on to say, It was almost unfair, not being able to live to the end after all she'd done. I think Rachel deserves more credit than we, and KA, were willing to give her. She's a smart girl who would have found a way to live life and just deal. Its what we do well as humans. Adapt.