I think that another class would be great if they handled it right. I know a lot of people are concerned about balance issues, but if they can balance nine classes I think they can handle ten.
Regarding the spy and sentry changes, I more or less agree that the current system could use some improvement. Though I have to say that it's great to have enemies blasted with your level 3 sentry, it's a pity that the sentry is defenseless once you leave it and a spy/ demo finds it. I think the disguises are pretty fun to use if it works right, but so many things can go wrong, that it's become obsolete to all but the most lucky and skilled players.
I've always thought that a melee dependant class would be awesome. He could be a rennisance(?) nerd that wears heavy armor and can parry attacks, or maybe a ninja or samurai to satisfy the weeaboos and even out the racial demograpics. I've also thought that an Indiana Jones rip-off would be hilarious/badass. He could be the Hero, and carry around a grappling crossbow for manueverability; also, his melee weapon could be a pistol. He could pistol-whip people, and only use it for ranged attacks during the taunt. It would work like the Heavy's taunt, but it would only kill if an enemy nearby is taunting. (Remember that scene where Jones shot the swordfighter?)