ISTR that in Terrance Dicks's script Benton was supposed to be the sergeant who has to identify the Doctor for Crichton at the end of the scene (continuity mangling though that would have been).
Well, that really only would have mangled to continuity of "Mawdryn Undead," which has so severely mangled the continuity of nearly everything else it touches (and is rubbish to boot). I think that ship has already sailed.
But then, it could still have been in continuity if Benton had been a civilian in that scene like Lethbridge-Stewart.
In some cases, we don't have a lot of great choices left. But here goes:
One-- Ian. (I agree that the age juxtaposition would make this one more interesting than Susan. I keep having this vision of a recast 1st Doctor sitting on a bus bench next to Ian, the two of them slowly realizing who the other one is.)
Two-- Zoe. (As iconic as Jamie is, I somehow feel like Zoe would be a better fit.)
Three-- Not a lot of good choices here. I wouldn't mind Yates, Benton, or Jo. Barring that, bring in Lethbridge-Stewart's daughter.
Five-- Nyssa. (Or maybe Tegan. Tegan was more interesting generally but Nyssa had much more chemistry with the 5th Doctor.)
Six-- Frobisher. (Because the show needs more talking penguins generally.)
Seven-- Bernice Summerfield. (She can call out River Song for being a total rip-off.
) Or maybe that Nazi scientist from "Colditz."
Eight-- Iris Wildthyme. (It gives us a way to use Katy Manning without bringing back Useless.)
Nine-- Captain Jack Harkness.
Eleven-- Rory. (I think it would be funny if, in retrospect, Rory realized that he'd met several of the Doctor's previous incarnations before while living as the Lone Centurion. Like, when he sees the 1st Doctor, "Isn't the Maximus Pettulian, the famous Roman lyre player?")