Assuming the quincies are the main villain of the arc, of course. The common approach by authors is to reveal the apparent main villain early and then to reveal a puppeteer later on who's the real villain. At that point, the original apparent villain ends up becoming an ally (sometimes only tense and temporary) of the protagonist to deal with the common foe.
On the subject of quincy foreshadowing, there's been a potential for a while.
When we were first introduced to the quincy, Urahara told Rukia (who passed it on to Ichigo) that all surviving quincies hate the shinigami for the masscre 200 years ago. If Uryuu and Ryuuken are the only two shinigami left and they don't care about the massacre then exactly what 'all surviving quincies' was Urahara referring to?
When Uryuu fought Mayuri, Mayuri observed there were techniques he had read about but in the quincies he studied he could never witness them. Uryuu was the first time he saw those techniques. Mayuri also knew absolutely nothing about Sanrei Glove. He hadn't even heard of anything like that. As a result, while that fight made it clear Mayuri was probably the shinigami expert on quincies, it also made it clear that he didn't know as much as he thought he knew and that the quincies he studied clearly hadn't revealed the full might of quincy power to him (whether because they were dregs or had the will to hold back, who knows).
Souken also told Uryuu that the reason Ryuuken gave up the quincy way was because he had something to protect and his sense of justice was defined what it was he had to protect. He also told Uryuu that only when he could understand his father's heart would he understand what it was he needed to protect. He also said that when Uryuu understood his father's heart he would question whether or not he wanted to continue being a quincy.
Those were the stipulations for using the Sanrei Glove. Souken gave Uryuu a power in trust for the day when he learned the truth about his father's heart, faced the question of whether or not he wanted to continue being a quincy and, if he decided to continue being a quincy, he would then face trials that would require the use of the glove.
Ryuuken told Uryuu his power was immature and that if he had waited for his power to mature he wouldn't have needed the glove to fight Mayuri.
Souken also told Uryuu that his desire was for quincies and shinigami to fight side by side. In response to Uryuu asking him if the shinigami were at fault for wiping out the quincies, Souken replied that it was better to work at a way to ensure it never happened again. Well, if one old peacenik, one retired quincy and one novice are all that's left how on earth could the quincy/shinigami war ever happen again for Souken to be so worried about avoiding a repeat of history?
And one more thing. Souken's death. Hollows came to attack Souken in a way that left him out-numbered. He died. We know why the shinigami didn't arrive in time to save him but we don't know why he was attacked by a gang of hollows in the first place. More importantly, he gave the Quincy Cross to Ryuuken, insisted on it, from what Ryuuken implied (Ryuuken said it was against his wishes that he was holder of the cross and what it signified). So, if the attack on him was as suddenly and random as Uryuu acted like it was, exactly when and why did Souken force the cross onto Ryuuken? Did he know he was going to die?
The manga has raised plenty of questions about the quincies that the appearance of powerful revenge-filled quincies could answer.
