Maybe about how Peridot failed her mission, joined the rebel team, and called YD a clod? And/or maybe about YD plotting revenge and destroying the earth.

That's a good idea!

A heartbreaking mourning song about the death of her friend/sister PD because SU will make you feel sympathy for all of the characters.

A true villain song, like Scar's one in the lion king ?

Maybe one were she laments on how the other Diamonds changed after PD died, maybe it's go like this

Yellow Diamond: It used to be so much fun around here

She didn't have a care in the world

Blue Diamond wasn't always so cruel...

White Diamond was beloved by all!

Oh Pink Diamond, poor small diamond...

You were taken from us far to soon

Perhaps if I had just been a little more caring towards you...

No time for this, that planet will FALL!

The ones who took you from us will die!

(She leaves the room, her Pearl begins to sing softly)

Once upon a very long time ago...

My Diamond threw parties for every colony she made

Once upon a time Blue Diamond smiled...

Once upon a time gems knew who White Diamond was

That planet must pay! 

And then maybe, just maybe, she will smile and laugh...

Yellow Diamond: Soon Blue Diamond I'll avenge her

Soon White Diamond you will no longer feel sad

Soon the ones who took her will pay!

Yellow Pearl: Soon I'll hear her sing once again

Soon she'll reopen her colonies

Then soon my Diamond will laugh once again...

Both: Soon that planet will...pay...

I feel like it would be a crossbetween a villan song and a sad song. Such as YD singing about PD.

I'm just dying to know what is going to happen. I bet that in this song we'll learn more about YD and her Pearl. But, yes it could be about PD.

It'll be all this and more, just you wait! SU songs are always quite thought out and usually meaningful, perhaps we'll get a sonet about perspective, or an elegy about destruction; or maybe we'll get a riveting banjo tune!

Honestly idek anymore. There are so many types of villain songs that it could be anything