5. William Russell Is Still Alive
So, I'm sure I lost all the Tennant fangirls already by going back to 1963. But hey, it's the 50th anniversary! And anyone who DOES recognize Russell as Ian (who, with Jacqueline Hill's Barbara and Carol Ann Ford's Susan, was one of the original TARDIS crew in 1963) will probably nitpick that the aforementioned Carol Ann Ford is still alive too. But here's my take on this. Susan comes with a lot of backstory (She was the Doctor's Granddaughter, whom he left to find her own life rather than get tangled up in his messes.) As a Time Lady (Yes, that's a thing) she also would require a "So what happened to you that lets you survive the Last Great Time War/Why didn't the Doctor check up-save you/Why are you a thing NOW?" story that would be so continuity heavy that I just can't see it. Ian (and Barbara, for that matter) were returned to Earth. They lived a (relatively) normal life after leaving the Doctor, which was neatly canonized in Russell T Davies' episode "Death of the Doctor" in "The Sarah Jane Adventures". So, he's been on Earth, and would pretty much be the age he is now. Russell himself, for those who don't know, is a stellar actor who's been turning in substantial audio performances for Big Finish as Ian (and others, when needed). He clearly still has a great love and respect for the show and its fans. A showing for Ian wouldn't need to be intensive or plot-essential. Simply, he could vouch for the Doctor when an earth crisis requires him to win a nation's trust (or the world's) for a risky plan (He is apparently a respected Professor). He could turn out to have something the Doctor needs, and a stop over to Cambridge in the Tardis is now no problem (In Ian's day, the navigation was a bit wonky - he was lucky to get home two years late). Or on a bittersweet or reflective note, the Doctor could stop out to see him in the event of Barbara's passing. (Which, really, is two tributes in one - three if he alludes to Susan) The last scenario could nicely set up/precede any main plot thrust, too. Really, the bottom line is this - We're celebrating Who's 50th anniversary. Russell was there from the start, and bringing him in even for a brief scene as Ian is entirely do-able. As Elizabeth Sladen, Caroline John, and Nicholas Courtney's passing have shown us, we're lucky to have him, and the time is absolutely right. Plus, the show OWES him the opportunity. The aforementioned 20th Anniversary story "Mawdryn Undead" would've seen Ian returning as a teacher the 5th Doctor met and then consequently sorted out some alien-Time Lord disagreement with. Russell's unavailability saw the Brigadier from UNIT show up instead, and also saw him awkwardly explaining that he teaches math now at a boy's school. Uh huh.... Courtney does a great job working with what he's given, but Ian would've made a hell of a lot more sense. He's here now, and the 50th is too.
What I'd Settle For: If you're not giving the Doctor the opportunity to say "Chesterton!" once more (And COME ON, Matt Smith already does that sort of thing!), then cast Russell in Gatiss' "Origin of Doctor Who" drama as a BBC suit or something.
What I'm Afraid We'll Get: River Song. In this particular year, that'll be like unwrapping socks on X-mas morn.