We obviously won't know anything with certainty until Thursday, at least, but this just occurred to me, so I thought I'd share.
In the promo for "When it Rains," Peridot is shown to fear a rainstorm, stating "It's happening!" "It" is likely referring to the emergence of the Cluster, which suggests a storm is involved in the latter. Now, where have we heard about a storm before?
The Geode. Pearl stated that it contains a "synthetic storm"- but what if, unbeknownst to the Crystal Gems, the Cluster is incubating inside/beneath it? This would also account for why it had started cracking in the first place- the Cluster wasn't fully mature yet, but it had nonetheless started hatching. When it hatches, the Geode will break, releasing both the storm and the Cluster.
Thoughts?