So, by adults, I assume 20-25 age range? College/University age? I'll do this in the order the books are narrated, then.
Jake:
I sort of agree with the consensus that he wouldn't be as ambitious as an adult prior to the war. I can buy the middle-management thing. Or, hell, maybe even just a student--his dad's a pediatrician (which, I assume get paid almost as well as full Doctors) and his mom's a writer. Of course, Jake could have a work ethic to him--not wanting to just coast on his parents' money. I read a fic once where Jake was in the military before encountering Elfangor--which would fit his family history, having a grandfather in WWII and a great-great uncle in the civil war. I think I could settle for either middle-management nobody, or active soldier on leave at the time this series begins. I can also see him dating Melissa Chapman in High School, and maybe afterward; likely as a second option, since he couldn't get the courage to ask Cassie out while they were in school--again, assuming we're going with them knowing each other previously.
Rachel:
Lawyer, totally--or studying to be one. She's a lot like her mom, and she admitted to being just okay at gymnastics, so that wouldn't be a career option. Or she could get into her dad's business-- she could totally pull of news anchor, I think--maybe even a field reporter investigating...the Sharing...Hang on, I think I've got an idea going here...* I've often wondered how Rachel would function if she were allowed to be "sexy." We couldn't see that in the series, since it was for 9 year olds. But I can see her using sex as a substitute for the violent nature she'd develop once they enter the invasion. She'd be dominating--but not a dominatrix-- and wouldn't keep a long-term relationship; instead using guys as a means to an end--scratching an itch, if you will.
Tobias:
Poor, poor Tobias. We just have so little faith in you
I've played with the theory that he'd become a Sci-Fi author--but that was in a world where there were no Yeerks (but Tobias still existed...) and the Animorphs went to a high school reunion...That was a fic I never got around to starting. In this series, where they're older, I can see him as an intern, or low-ranking employee: maybe a mail room job, or, as you've all said, a janitor. Or maybe he could work as a camera man, or intern at a news station, where he follows around a curious, and attractive, local journalist and field reporter investigating the Sharing...
Cassie:
Yea, we're all pretty much going the Gardens route. So, why beat a dead Andalite?
Marco:
I see him maybe working software, or at a video game store, or something. He doesn't seem like the undergraduate type--probably didn't go to school after high school. In the fic where Jake's in the military, Marco was an auto-mechanic or something like that--which I can buy. Maybe he'd be a programmer like his dad? The thing is: Marco's smart and all, but he's just as motivated as Jake--which is to say: not very motivated. I could see him working construction, too...Hey, don't the Animorphs have one of those in their town? Maybe Marco noticed some people snooping around there at times?
Ax:
Yea, aristh or slightly higher up than that. It would be interesting to see a power-struggle between Ax and Jake at first. I figured Ax would just let Jake run things since he new the territory, and the Animorphs were the last ones to see Elfangor alive, and he was...I don't know, the Andalite equivalent of touched...that they would fight for Elfangor's cause.
David:
...Wait, he wouldn't appear here, would he...
This brings up a good point: what happens to all the secondary characters? Tom, Melissa, David, Chapman, etc. How do characters that the Animorphs new in high school fit into this new world?
* I can see Rachel as a reporter chasing a conspiracy, and that's how the s**t hits the fan in this series.