I personally don't think the characters were all that lame, nor do I think Jimmy should be the antagonist.
But Bully needs to be harder - definitely.
I think by gay, they mean the bisexual characters. Frankly I couldn't care less... it's optional. If they bug you that much you can at least hit them instead. Both Trent and Kirby tend to recieve loads of beatings for this.
As for the difficulty, as long as the difficulty is real and not fake, yeah sure. Alternatively difficulty settings and I think at the end like I've suggested with 'the hole' fighting arena becoming available, fights could be made insanely hard there.
Jimmy as the antagonist would make an interesting idea and its something I've considered. After all, he ultimately achieves nothing except for himself at the end of the game. Nobody bullies him but the level of bullying is practically identical to when he starts and is therefore he could be seen as a cause rather than a cure seeing as he's a bit of a jerk. Still I think the plot would be better of ditching Gary and Jimmy. Gary and his character design process was really flawed in my opinion (yet he's still ridiculously popular... sigh) and bringing him back would be a complete disaster. There's just no way to write him out of the hole they started him in without it being really stupid.
I've always thought if Bully 2 was to remain at Bullworth that whether Jimmy is there or not (and he absolutely should NOT be the main character whether its at Bullworth or not... don't dislike him but I've had enough of him) the problems that occured there should ultimately not be resolved despite there being a 'feeling' they should have been. Of course I've got my own take on Bully 2 which I started coming out with and will continue one of these days... lol.