Mage will still be strong, every class has their obvious counters, but that just means it takes that extra effort to outplay them. I get that RNG resists are cancer.
To be fair, TBC destro is so much more fun than SL/SL. You can go full Drakedog on ppl. Destrochaining is so satisfying.
Rogue is gonna be crazy fucking good. I can already see some go mutilate meme spec and try to 2-shot people (not best spec, but fun and gimpy).
Boomkin has always been TBC sleeper spec. It was quite complex overall, so no one really knew how to play it properly back in the day. However, today's playerbase is way more experienced, and I can guarantee you will see some killer boomkins in the 69 bracket as well as endgame.
The good thing is that literally every class is viable. Some specs will require more effort, but no matter what you play, you'll be fine. Even if you wanna play fkn feral.