I’m thinking that Donna is going to be revealed as something other than a “temp” that the Doctor just happened to meet twice (the end of this last week’s episode certainly laid the groundwork/telegraphed such a reveal). So here’s who (what?) I think Donna is…
She’s The Key to Time.
Huh, you may say? What’s that? Well, WAYYYY back when Tom Baker was playing the Doctor an entire series of episodes were structured around the Doctor’s quest/search for the Key to Time, an artifact that allowed the owner to have complete power over time and space. The last of the six pieces was revealed to be a human female. In order to prevent the power from landing in the wrong hands the Doctor threw the pieces across time and space… Lots of detail on this story arc is here.
Knowing how much the producers of nuWho love to have a near-apocalypse scenario be the challenge the Doctor has to overcome at the end of each series (and how often they have pushed a big ol’ “reset” button to save the universe) it would not surprise me to see them pulling the Key to Time back into the show’s “timeline.” This will mean, in all probability, that Donna will either A) die or B) forget everything after she “fixes” things… I wager B.
As to “who dies” in the finale… I’m thinking it’ll be Captain Jack, just to shock everyone (because he “can’t die”) and because they plan on making Torchwood more “family friendly” next year…
Anyway, we’ll find out in two weeks…
UPDATE: I was completely wrong. Ain’t the first time, and it’s won’t be the last.