And who do we know that suffered a complete personality shift accompanied with the inability to do one's rather important job on account of obsessive love... Here we also have a huge potential plot-point that seemed very much to be left undeveloped.
Meanwhile Remus Lupin, who was thinner and more ragged-looking than ever, was sitting beside the fire, staring into its depths as though he could not hear Celestina's voice [singing a song called 'A Cauldron Full of Hot, Strong Love'] (p. 310)
There's a lot going on in this scene, and the song lyrics are saying something. But what are they hinting at? They're seemingly speaking of love tampered with magic. Or are they mirroring Lupin's inner landscape? "Oh, my poor heart, where has it gone? It's left me for a (stunning?) spell..." Lonely, heart-broken, and being insistently propositioned in this state? "I'll boil up some hot, strong love to keep you warm tonight."
"And incidentally," said Snape [...], "I was interested to see your new Patronus." [...] "I think you were better off with the old one," said Snape, the malice in his voice unmistakable. "The new one looks weak." (p. 153)
The Order uses Patronuses to communicate because they can't be faked. Snape is suspicious for some reason. Harry, as ever, misinterprets Snape. Tonks is not wounded by the words, like you'd expect, there's 'shock and anger' on her face before she is 'covered in darkness once more'. (p. 153) Is it Patronuses they're talking about, or sides in the war? What does Snape know?
['Why would your Patronus change?'] Lupin took his time chewing his turkey and swallowing before saying slowly, 'Sometimes... a great shock... an emotional upheaval...' (p. 319)
A great shock, an emotional upheaval -- like having a crush? Or maybe having someone close to you die? "But Tonks and Sirius hardly knew each other!" said Ron. (p. 93) Remus is extraordinarily careful in answering Harry, but is it because of what he knows, or is it that he suspects?
Lupin has been underground, spying on the werewolves, nearly all of whom are on Voldemort's side, but they do not trust him. (p. 313) Would not Voldemort know what he was trying to do among the werewolves, if he stuck out like a sore thumb among them? Lupin has not been in contact with Tonks, but Tonks has apparently been trying very hard to get into contact with Lupin. Enough to even harass his mission?
Tonks has drawn away from her family (p. 319), Tonks and Draco are being consistently described in similar terms. Harry meets Tonks outside the Room of Requirement 'as though she frequently strolled up this corridor', which is on the other side of the Castle from Dumbledore's office, which was her stated business for being there. And then there's this: "Nothing in particular," said Tonks, picking, apparently unconsciously, at the sleeve of her robe. "I just thought he might know what's going on." (p. 436) Left sleeve, was it? Who do we see 'unconsciously' fiddling with their arms? In fact, who do we see throw a hissy-fit over his sleeve earlier on in the same book?
Bellatrix does not trust Snape (ch. 2) to come through for Draco. And what has Hermione to say? 'Well, [Tonks] was fighting Bellatrix Lestrange, wasn't she? I think that she feels that if only she had finished her off, Bellatrix couldn't have killed Sirius.' (p 94) The woman was a fully trained Auror, fighting someone freshly out of a decade of debilitating imprisonment. We don't see much of Bellatrix after the second chapter, but would she have been content to just let things lie? Be satisfied with the Vow she saw Snape flinch taking?
And then there's the scene by Bill's bedside. Very suspect, almost inappropriate behaviour from Tonks. She seems not only interested in how exactly Dumbledore died (and not an iota in Bill's condition), but also very interested in Snape, his part in it, but particularly his place in the Order. '"I'd love to know what Snape told [Dumbledore] to convince him," said Tonks.' (p. 574) Love? That's a very strange choice of word.
And note Lupin's behaviour, as well. Earlier he had expressed the same kind of unwavering trust in Snape that Dumbledore had, even defending him to Harry, and yet he is very quick on turning against Snape and he's suspectly quick at coming up with reasons how Snape might have accomplished his hoodwinking of Dumbledore. This is a man who has seen a lot of death and suffering in his life, and yet, grief-stricken 'Lupin collapsed into a chair beside Bill's bed, his hands over his face. Harry had never seen Lupin lose control before' (p. 572) The last straw? Dumbledore seems like a man who would have arranged more than a simple last will and testament for the eventuality of his death.
There's bound to be more to Book 7 than the Hunt for Horcruces and the history of Regulus.