Mundungus Fletcher
Mundungus, funny though he may be, is a pretty scummy character. Although he's a member of the Order of the Phoenix, he's a petty thief [OP-5], and he plies his trade pretty diligently throughout the books. So diligently, in fact, that he can't even be trusted to stand guard reliabily [OP-1].
We learn that Mundungus is personally loyal to Dumbledore, who rescued him once [OP-5]. That loyalty, however, doesn't seem to extend to anyone Dumbledore cares about, given that Mundungus's devotion to his trade takes him back to 12, Grimmauld Place to steal items Harry has inherited after Sirius's death [HBP-12].
Somehow Dumbledore stops Mundungus from doing so [HBP-13], and Mundungus goes into hiding. Not very well, apparently, or maybe the lure of thievery was just too strong for him, as we learn he's been thrown into Azkaban for impersonating an Inferius during a burglary [HBP-21].
Well, it's a good thing we know where he is, because I wouldn't be surprised if Harry might need to have a chat with Mundungus in Book 7. At some point, Harry will have to remember the locket he saw during the cleaning up of 12, Grimmauld Place, and he'll have to remember that Sirius's brother was named Regulus, and he'll have to put the two together and wonder if that very locket mightn't be the locket Horcrux "R.A.B" stole from the Cave.
And what if he goes back to 12, Grimmauld Place and finds the locket missing? Well, first thing, he should check Kreacher's stash. But if it's not there, I'd say Mundungus is the first stop in the search for the locket.
And one more set-up about Mundungus, who's already someone we really wouldn't want to trust with our lives. We know he joined the Order out of loyalty to Dumbledore. But where do his loyalties lie now that Dumbledore is dead?
Given all the secrets of the Order that he must know, maybe it's a really good thing that he's in Azkaban.
Rufus Scrimgeour
We first met Scrimgeour in Half-Blood Prince [HBP-1]. But we were actually introduced to him back in Order of the Phoenix, and it wasn't an introduction that should make us feel comfortable with him. We learn, in a throwaway line, that Scrimgeour is suspicious of the Aurors who are members of the Order [OP-7].
That suspicious nature continues once Scrimgeour comes into power. We see early on that he and Dumbledore are in disagreement [HBP-4], and Scrimgeour overtly sets himself against Dumbledore every time we see him after that.
We know that Scrimgeour, cynical manipulator that he is, wants Harry to serve as a public relations shill for the Ministry. We also know that he wants information that he can't get, specifically about Dumbledore [HBP-16]. (Just think how many avenues he must have tried before dragging Percy to the Weasleys so he could corner Harry!)
And we know that Harry's got his number:
"...You never get it right, you people, do you? Either we've got Fudge, pretending everything's lovely while people get murdered right under his nose, or we've got you, chucking the wrong people into jail and trying to pretend you've got 'the Chosen One' working for you!"
"So you're not 'the Chosen One'?" said Scrimgeour.
"I thought you said it didn't matter either way?" said Harry, with a bitter laugh. "Not to you anyway."
"I shouldn't have said that," said Scrimgeour quickly. "It was tactless--"
"No, it was honest," said Harry. "One of the only honest things you've said to me. You don't care whether I live or die, but you do care that I help you convince everyone you're winning the war against Voldemort..."
What will we see from Rufus Scrimgeour in Book 7? More of the same, I would expect. He has clearly drawn battle lines between himself and Dumbledore, just as strong, perhaps, as the lines drawn against Voldemort. And as Harry has made it quite clear that he is "Dumbledore's man through and through" [HBP-16, 30], undoubtedly Scrimgeour will continue the adversarial relationship against Harry.
However, with all the storylines to be fulfilled in Book 7, I have to think that any obstacles thrown up against Harry's quest on the part of the Ministry of Magic will play minor roles compared to the quest itself.
Ollivander and Florean Fortescue
Both Ollivander and Fortescue disappeared at the same time [HBP-6]. But there was a big difference between them: Fortescue, based on the state of his shop, was clearly dragged off kicking and screaming. Ollivander, on the other hand, left his shop in good shape, with absolutely no signs of any struggle having occurred. So one must ask: Did he leave with the Death Eaters voluntarily?
In any event, as Mr. Weasley points out, it's not good for Voldemort's side to have Ollivander in their clutches. As the best wandmaker around, he could be uniquely valuable to them.
I expect that we will see Fortescue again, probably at the end of Book 7, as part of the celebration of Voldemort's defeat. Ollivander... I'm not so sure about.
Wizard qualification
I didn't really have another category to put this topic in, and as I only have a brief thought about it, I thought I'd just slip it in here.
We know that coming of age and being "qualified" as a wizard are two different things. It seems that one becomes qualified upon taking one's NEWTs. Hagrid is certainly "of age," but, having been expelled from Hogwarts, never became "fully-qualified." The Weasley twins, having dropped out of Hogwarts, presumably are also unqualified, despite their extreme skill as wizards.
So Harry, who has announced his intention to also drop out of Hogwarts, will also presumably not be a "fully-qualified" wizard. Which makes the following statement by Dumbledore (said at the Cave) of interest:
"Voldemort will not have cared about the weight, but about the amount of magical power that crossed his lake. I rather think an enchantment will have been placed upon this boat so that only one wizard at a time will be able to sail in it."
"But then--?"
"I do not think you will count, Harry: You are underage and unqualified. Voldemort would never have expected a sixteen-year-old to reach this place: I think it unlikely that your powers will register compared to mine." [HBP-26]
So Harry slips through under Voldemort's radar, as it were, because he is underage and unqualified. Now he will be of age, of course, when (or shortly after) the final book begins. But will it make a difference that he is unqualified? Will that somehow help him to avoid any of Voldemort's traps?
One can only hope...
........
That finally wraps up my thoughts on set-ups and payoffs regarding the various wizards of Harry Potter. Obviously I have cherry-picked among the wizards and haven't covered everyone. (Feel free to weigh in with thoughts on wizards I should have touched on!)
Next... On to the most fascinating wizard in the whole story... the Half-Blood Prince himself, Professor Severus Snape.
