Zygons? Seriously I know you'll say you don't think they'll be in it much, but they will still be in it, and they are classic era monsters, so clearly there is at least one element of the classic series in this special.
Not much different from the episode that will air this Saturday.
I think the other issue to consider here is that just because no classic Doctor has been approached to be in the episode it doesn't mean they won't be in it. I mean, Shatner and co weren't approahced to be in Trials and Tribbleations either.
And it isn't like Moffat has completely ignored past Doctors. He showed them all in the Eleventh Hour, had Smith using the first Doctor's library card, heck we even got mention of a grand daughter last week!
It's the 50th Anniversary, not just another regular episode. I'm no advocate of an hour of cameos but there should be something more than, say, having someone wear Tom Baker's scarf* to acknowledge the contribution of the people involved in the first 42 years of the series. And no, old clips aren't good enough either.