While we’re waiting (and waiting, and waiting) for season 3 of “Downton Abbey” to air in the States (the show began airing in the U.K. in September, but won’t cross the pond until January), we’ve tried our best to avoid spoilers. Still, we already know that Shirley MacLaine will be playing Cora’s American mom in upcoming episodes, and now Julian Fellowes has announced that he has some other exciting guest spots planned for season 4. Of course, we can think of a few established actors with the acting chops to add to the understated yet gripping drama. Some might seem obviously, while we suspect you’ll find a few others unexpected at best. Here’s our top ten, offered in what we hope is a most appropriate fashion.