A lot of people (and the wiki page) are explaining the ritual from the beginning of Super Watermelon Island as a way to select a sacrifice. Which, obstinently, makes sense, as this is what happens. But I don't think the ritual was like "nose goes", as many people have suggested. Instead, I think the purpose was to identify Steven.
Fact 1: The ritual happened as soon as Steven arrived at the town plaza. Either this sacrifice happens way too much, or it happened as a result of his being there.
Fact 2: Every melon knew what to do during the ritual. Only Steven, being the outsider, was unsure, which led to him being chosen. What happens when they all know what to do? The process wouldn't work. This strange, simple gesture the melons do seems perfectly designed to say "not me".
Fact 3: They dressed Steven as Steven. The yellow star & pink roses are all symbols he is associated with. They likely see Steven as some sort of god, (in fact the entire mythos of the watermelon stevens is fascinating and deserves investigation) and treat him as such, carrying him around on a chair. If this sacrifice was meant for anyone, why the specific garb?
Fact 4: Malachite reacted quite violently to seeing Steven. This implies that this is not a regular occurence, or that Malachite has a very short memory.
Fact 5: The ritual seemed to do the one thing it needed to do to save the world: inform (their gods) Steven & the Crystal Gems about the impending release of Malachite.
I would guess that, opposed to ritual sacrifice, this process was explicitly designed to identify Steven when he possessed his avatar, and take him to the horrible demon that lives near the melons. This implies some sort of cognizance of Steven's abilities, the possession process, the ultimate fate of Malachite, etc. It also makes the whole ritual sacrifice thing seem a lot less horrifying, given the rarity and necessity of its occurrence.
Thoughts?