Completely unfounded theory of the day:
The chest inside Lion’s mane always gave me an old Zelda vibe, you know? And the show does have a lot of influence from Zelda and other videogames.
One of the classic videogame plots is the “collect the pieces of the broken magical artifact” plot. So, with this idea, what if what’s inside the chest isn’t Pink Diamond’s shattered remains… or well not all of them, but only one shard, and they have to go find the other shards in order to piece Pink Diamond back together? starting an arc about finding all the pieces of Pink Diamond´s gem.
Gem shards have been a big element in the show. Starting with Pearl looking for a missing shard in Frybo; Pearl, Amethyst and Steven joining to look for the lost shards in Secret Team (when we learnt Rose had tried to collect the shards of the fallen Crystal gems); The Cluster arc, when we learnt Homeworld collected the shards to experiment with them. What if all these were foreshadowing for a future arc?
Why would they want to piece Pink back together? I’ve heard the theory that Pink Diamond is the key to undoing the corruption(whatever it is, it was caused by the Diamonds, it wouldn’t be too farfetched that to undo it they will need a Diamond or the power of a Diamond’s gem). Let’s go with that, after the corruption happened, Rose tried to collect Pink Diamond’s shards in secret, but she was never able to find them all (Keeping with videogame parallels, It would be like in Castlevania 2: Simon’s Quest. Simon Belmont killed Dracula at the end of first game, but now he is forced to collect the pieces of Dracula’s body to resurrect him again in order to break the vampire’s curse).
As if this theory wasn’t enough of a stretch already, I’m gonna give some guesses as to where are some of the other missing shards:
The Palanquin: Some of her remains are still in the place where she was shattered, this should be the place to start looking.
Lion: Lion is a Pink Diamond shard that took form, or he ate a shard (we know he likes eating some weird stuff)
Pearl has one hidden (possibly in her gem, like Lapis’ mirror): I have no theory as to why she would hide it, but it’s clear she has a secret about Pink Diamond still yet to be told (and what was she doing with those shards in Secret Team anyway…)
The Cluster: Some of Pink’s remains got mixed with those of the Crystal Gems. Now part of her essence is one with all of them. Oh the irony…
Blue Diamond: She keeps one of Pink Diamond’s shards. As we know she is dedicated to preserve everything that has to do with Pink. She probably keeps it in secret from Yellow, who would prompt her to get rid of it or even destroy it.
Other treasure chests we have seen around: