I think I know why the classic doctors weren't included: Their age. If it was an adventure show (like the 20th anniversary show--with Baker in his velvety-smooth pimp hat lol) the writers would have to explain why the classic actors look so much older. That said, as an American viewer, I have no idea what has been planned. Baker and Davidson would still would be fun to watch, though.
And that's why I don't want most classic doctors involved. But McGann is still just about in his prime. It was only the mid nineties when he was the doctor, and I think he's aged well. If they had Troughton with grey hair in the two doctors, then why not McGann with just like a few bags under his eyes and a couple of lines if that.
I think McCoy is still useable too. Yes he's aged since 1989, but remember that he aged quite lot in the 96 film, and I think since then he's not aged too much.
Baker, Davison, and C. baker have aged too much since their last appearance though, especially Colin. He's aged terribly. Tom's not aged too badly considering he even began to age by the end of his tenure as the doctor, if only he just lost his weight and put on a curly wig, he'd be good enough for the role. But I can't see that happening. Davison's not aged too badly, but because he was so young when he was the doctor, in comparison he's too far gone.
But really at the end of the day although I'm very happy to see David come back, I'd be so much more than happy to see Smith, Tennant, McGann, and McCoy team up for a "The Four Doctors" special. I've come to terms Eccleston will never respire the role.