For example, maybe the Doctor won't save Rose next week - that would be a good reason for inserting that speech. After all, the final episode is entitled The Parting of The Ways. With the Doctor and Rose as the established team, that title has an obvious interpretation, but with the menage-a-trois of the Doctor, Rose and Captain Jack, it's a bit more complicated.
There's also speculation here that next week's episode will be the regeneration story (or at least lead directly to regeneration). I think that's plausible; to me it seems more likely that we'll have a season-end cliffhanger (particularly as we know there's going to be a Christmas episode and a second season) than that everything will be wrapped up neatly.
It seems clear that next week's episode must be to do with the Time War; but there are too many loose ends that haven't been tied to that yet. Captain Jack's Time Agency and missing memory has nothing to do with the Time War, as far as I recall, so what about that? Didn't he lure the Doctor and Rose with his 'Bad Wolf' bomb in The Empty Child?
As for the Doctor being the Bad Wolf ... maybe that's no bad thing. The Game Station Controller had the mind of a child; Who's afraid of the big Bad Wolf? is a nursery rhyme; and who is afraid of the Doctor? Well, the Daleks, of course - except that they have emotion removed from them.
Hmm, this is becoming unhealthy. Time to stop speculating and go out into the garden.