The Stig, regardless wether you believe Tennant/Piper are an important part of the celebration, and regardless wether you see the need to bring in Classic Who actors, DalekJim is correct about the Anniversary celebrating An Unearthly Child. This is the 50th Anniversary, so, it absolutely is celebrating the first episode, there is no denying that, it's a simple fact.
There would be no danger whatsoever to the ratings to include William Russel. Ian could easily be in the thing without confusing the General Audience, who may or may not know who Ian is. It would take very little, no more than seconds or a minute to fill in who he is
Certainly. If the new series could bring in Sarah Jane Smith in "School Reunion" with a minimum of confusion, I don't know why they couldn't do the same with Ian. Heck, maybe Ian gets lucky and gets his own spin-off next.
Come to think of it, look at The Sarah Jane Adventures.
That's a show who's target audience wasn't even born yet when the original series went off the air in 1989. And yet the lead was a companion on the show in the mid-1970s and it often seemed to do a better job of bringing in old guest stars than the new Doctor Who
has. Both Brigadier Lethbridge-Stewart and Jo Grant made their reappearances on The Sarah Jane Adventures.
(And RTD mentioned once that, had the show lasted longer, he might have eventually brought back Ace too, possibly including a Sylvester McCoy flashback cameo where we see Ace leaving the 7th Doctor.)
Besides, anniversary specials are the appropriate time for self-indulgent fan-wankery!