I didn't forget anything. The Cataclysm can be resolved by defeating Xiang Liu, as Qu Chen explained last chapter. And the reunion could happen right in this arc as they fight Xiang Liu.
Achieving top cultivation is not important to the story. In this story, cultivation is just a means to an end. I think the story will end when A-ting is strong enough to fight Baili Zong.
Yes but Xia Dai is not afraid to use time-skips. We already had two time-skip. There could be another on after this arc. We could jump to where A-ting is already a Nascent Soul cultivator, which is just one cultivation level behind Baili Zong who is in the Astral Projection level. One level behind would probably be enough for A-ting to fight him, considering she's always stronger than her cultivation level suggests.
I have been thinking the same thing for a while now. It resembles the usual japanese shounen structure. At some point, there is a big evil, a big problem, a big "something" that is mentionned from time to time. And then, it arrives. In Fairy Tail, it was Zereph. In Naruto, it was the shinobi war. In Boku no, it is All for One. Bleach does the same, too. Point is, the world and story narrow progressively to the point of only adressing this one topic, and it becomes a pretty long story arc that entails many conflicts, many characters, many everything, as a "climax" of sorts, just before reaching the epilogue (this is where i tend to drop these stories, because i dislike obsessional storytelling that can only focus on one thing).
I know that here, this is chinese, not japanese, but i still can see this "heavenly cataclysm" saga being the last before the epilogue. I don't think the author minds much about leaving opened story threads by the end. After all, "Song of the Long march", by nature, won't solve everything either (actually, the end shouldn't solve that much, by design, as she doesn't want to change actual History). Like all the fundamental questions Cang Wuzi left when dying... They won't necessarily be answered. As long as there is a "temporary closure" (that may happen if Xiang Liu is defeated), no need to adress the deeper truth of the world within the story itself, as it seems to me that this is not the meat of this story in particular (it is about the twins).
I personally wish for this to continue for some time, because there are many matters i want to see adressed, beyond the cultivation/cataclysm situation. The author made our MCs into royalty. She even introduced A-Ting to her grand-mother, the general, and his girl. She spent some time on the emperor himself (her father). There is all this "injustice" theme that revolves around poor A-TIng. I would feel very let down if author did nothing with all that and just left things hanging as another "fact of life that doesn"t require solutions".
One unlikely theory i have for what may be coming is that, after reaching golden core and fusing the earth-element core of her sword with her own, she may retain aspects, part of the personality or memories of the former cultivator who left the core behind. Maybe she will just use the purifying rain to perform a large-scale resurrection technique to adress her regrets, or whatever... I don't even know if whatever-was-the-name-of-this-tree-cultivator-Qu-Chen-met-during-his-coma could use such techniques himself, but he was wood-aspected. So maybe.
