Katie Bell, for the past five books, has always been a minor character, more or less. While she may appear again for Quidditch games and Dumbledore's Army meetings (if they return), I have a feeling that she'll stay as a minor character. I'll be on the lookout thought.
Lupin can be both, you know. The cool-headed man taking place of Sirius. I think he'll still be his cool-headed man self, but not take Sirius' place.
It is sort of like saying that a cookie you just took out of the box will replace the one you have eaten. It can't though, because the cookie you have eaten was special and yummy, and the one you have taken out of the box will never be like that cookie (just like how no two snowflakes are alike), even if they resemble in any way. Besides, Sirius and Lupin have two different personalities. Would you say that the Chips Ahoy (chocolate chip) cookie (Lupin) could replace the Oreo (cream between two chocolate wafers) cookie (Sirius) in a box of Oreo cookies? It couldn't, because the Chips Ahoy cookie isn't the Oreo cookie, and it would lead to awkwardness between the Chips Ahoy cookie and the other Oreo cookies.
In other words, Lupin would never be able to replace Sirius, because Lupin is not like Sirius, and it would probably have a horrible effect on the relationship between Lupin and Harry. Lupin will probably become a father-like figure, but not take Sirius' place at the same time.

That had to be the worst analogy in the world. Comparing Lupin and Sirius to cookies.
I have a feeling Grawp will become a key character in contacting the giants and gaining peace with them. After all, small or not he is a giant himself, and will know how to win the hearts of his people.
I personally think that Fleur Felacour and Viktor Krum will help connect Hogwarts to the two other schools, which will form a resistance against Voldemort throughout the other countries in Europe.
Karkaroff is either A. Hiding, B. Dead, or C. Swimming in the beaches of Hawaii. My guess he is either hiding or dead, probably dead from the way Voldemort promised to kill and/or get revenge on the two that didn't come to his group meeting in the graveyard. J. K. Rowling said we would see Wormtail back in action in book six. Now where he has been...that's a good question.
I doubt Fudge will become a Death Eater, so chances are he'll slowly diminish from the books, his name never disgracing the pages again.
For the teachers, it depends on the person. Most of them seem to not be in harm's way, more or less, and they haven't been mentioned before really except by names passing, so I don't think they'll appear randomly. For Snape, McGonagall, and the teachers that teach Harry, including the Astronomy professor... only time will tell.

My mind is all mush now. That cookie thing really messed my whole nervous system up. Now, what do
you think?