Scarlett won't leave. They wouldn't had a soundtrack album to flog without it! Scarlett will end up regretting leaving Highway 65 and will want to start her career again somehow. How many artists does Jeff Fordham have these days??!
Rayna isn't marrying Luke, but she won't marry Deacon either. They are the classic will they/won't they couple. If they get together seriously the show jumps the shark.
Avery WILL get back together with Juliet, and then Juliet will find out she is pregnant... obviously to much drama. (They have a real life pregnancy to either hide / go with next season).
Rayna will end up signing the band of Avery, Gunner and whatsherface, er Zoey. This of course will course tension as Avery will be touring with Juliet at the time.
Gunner will fail for Scarlet again, as he tries to comfort her for walking away from her career and persuading her to stay in Smashville.
Juliet will take Scarlett under her wing, and end up being BFF with her as they bond over the mother thing and the fall out from Avery dumping Juliet a second time, and Juliet's pregnancy.
Juliet and Avery will probably end up back together AGAIN, when Juliet looses baby (she won't have it, as it will be too much of an arse for Season 4 unless Avery decides to give up his career to look after baby).
Luke and Teddy, will form an anti-Deacon alliance, and stand there all 'manly' and disapprovingly at his relationship with Maddie. There will be at least one punch up.
Maddie will become even more irritating and whiney spoilt rich teenagery. Deacon will inevitably miss some important concert she's doing, because Luke and Teddy stop him from going somehow (probably get him arrested for something petty which he is completely innocent of) and Maddie will through a massive strop over it and go running back to Daddy Teddy.
Will will be dropped. Gunner will do lots of running around after him. It will be annoying. But not as annoying as Layla weeping constantly. Jeff Fordham will resign her as she will get loads of sympathy and publicity... He'll probably shag her too. She wasn't that kind of girl before, but she'll become it.
Rayna will split with Luke over Will. She will want to encourage/support Will, but Luke will spell out the disaster it will be for her label. Rayna will get pissy over him being homophobic, but will have a PR disaster on her hands with Will, so will be upset with Luke for being right.
Zoey will just be annoying filler. Again.
There you go Season 3 written. Roll on Season 4! :D