I thought this was a naga Chrysalis at first.

It’s a trap!!

…This will end very well.

STOP RIGHT THERE CRIMINAL SCUM! YOU VIOLATED MY HUSBAND!  

Far better than what I thought up. But I was rushed, and I didn’t want it to be forever forgotten or dumped if I thought too deeply. :P

Wouldn’t make sense, Twilight knew a Cadence when she was a filly. You need to put the divergence (from canon) point further back in the timeline for it to work.  
I got it.  
Changeling kingdom needs to gather Intel on Pony kingdom.  
Replaces a newborn princess with a changeling to make use of her education / future position of power.  
Princess grows up and while not going completely native due to regular check ups from undercover changelings, develops attachment to ponies and seeks peaceful conciliation between the two races.  
While foal sitting for Twilight sparkle Chrysalis meets Shiny and… [Insert standard Cadence + Shiny relationship plot with the additional plotpoint of Chrysalis freaking the fuck out about how Shining would take the fact she’s not actually a pony.]  
The climax of this part would be Shiny proposing and Chrysalis revealing herself as a changeling since she doesn’t want him to live with a lie for the rest of his life.
 
 
And that’s all i got.

Alternate universe popped into my head.
 
Chrysalis made up the pony Cadence because she fell in love with Shining. Who Liek?