This idea has been dabbling in my mind as of late: after reading the Great Alicorn Hunt, and reflecting on some tendencies in this fandom to treat Celestia the same way other fandoms treat characters like Dumbledore and the Sandiame/Third Hokage. My mind often thinks of ways to look at fandom cliches in new ways, and this is my take on the subject. Can't say if I would explore it further, but I am not against anyone else taking a dab at it.

Anyway, the idea here is that Celestia is musing in the aftermath of the Canterlot Royal Wedding, about how this may mean Shining Armor may now be doomed to die like a mortal, leaving a brokenhearted Cadence behind. Luna, who unlike Celestia has not had the years (about as long as Cadence has been a royal) to plan these things, is a bit more realistic, believing there is still plenty of time.

This also takes place before Season 3, where one could simply suggest this conversation leads to the plans to cause Twilicorn.

It also implies that Cadence and Shining Armor haven't come to realize the implications of Mayfly-December romance. As far as I know that pretty canon.

Hopefully this meets my goal: of trying to present a Celestia who could work as an antagonist, but one who isn't evil, diabolical, and could be an interesting take on the character for others to use.