According to different sources, there'll be a two-parter episode coming up May 12th, then the next three Thursdays until June. Here's my theory on the first two, according to the trailer and other clips:
May 12th: Super Watermelon Island and Gem Drill
I feel like Super Watermelon Island will start off with the Crystal going back to the island where the Sadie and Lars episode (was it called "Island Adventure"? Can't really remember) took place, before Steven tries to tell them they took care of the monster, and warp there without Steven or Peridot. They then meet the Watermelon Stevens, which have cultivated into a village. Finally having all the gems in one place, Malachite rises from the ocean (with Jasper mainly in control) and tries to destroy the Crystal Gems. The Gems (excluding Steven) fuse into Alexandrite and fight with Malachite. They win the fight and succesfully unfuse the Gems, but are stuck on the island due to the warp pad suddenly not working, or the gems being too weak to warp back. I say this because in the trailer, it seems that when they switch to Steven and Peridot, the Gems are nowhere in sight.
Gem Drill: Meanwhile, Steven and Peridot are waiting for the Gems to come back and are anxious to be doing something. After hearing thudding and shaking, Peridot and Steven finally decide they need to stop waiting and take off in the drill on their own. Steven feels like something isn't right because they're going off without the Gems, and going off into the middle of the Earth without any precautions taken. After this, I'm not sure what would happen. Either:
1. Suddenly, the Cluster (or a fusion expirement) grips the side of the drill, halting Peridot's and Steven's progress.
2. Garnet (Since she's the only gem who can withstand heats that we know) grips the side of the drill, after tracking the drill's location, and comes to help.
or
3. A new gem we don't know grips the side of the drill, hindering Peridot's and Steven's progress.
That's all I have right now.
(What we really are)