So far we have:
Celestia: controls the Sun
Lun: controls the Moon
Cadance: universal beacon of love and hope
Twilight: universal symbol of friendship and magic
Let's say Larson has a field day and FOR SOME REASON, has the writers a make another Alicorn.
What domain/responsibility would it have?
Nyx not included.

Lustynia: controls your sex drive

Think about it bro, how do you think all those people popping in Equestria all of a sudden wanna get it on?

2131961 Fluttershy, princess of the animals.

2131966 Lol.

2131966
Some people claim Cadence can do that. Not my idea, I claim no credit for any flaming pitchforks.

2131974
That sounds like headcanon.
I'd accept it, though.

2131961

There have been rumors circulating that the offspring of Shining Armor and Cadance will be the first naturally born alicorn since Celestia and Luna (as opposed to Twilight and Cadance who "earned" their wings and horn, respectively).

2131961 Spike: alicorn of sins.

2131961
I would say Terra, princess of the earth. But that idea is an old one from an alicorn Applejack story.

2131966

What about Lunesta? That would explain why every visitor gets knocked unconscious every other chapter.

2131961
They already have! Her name is Skyla! I think.

2131995

I actually had an idea a few months back about a princess made of crystal sowing the seeds for all the gems that seems so plentiful in equestria. unfortunately, I haven't yet had the chance to use the character, or even flesh her out.

The Staliion that is strangley obsessed with jelly. No muffins will be sold without Jelly.

2131961
Alicorn Derpy: universal beacon of confusion and accidents

2131961 Skyla (Cadence and Shining's daughter): Princess of the Goddamn Babies.

2131961
Never finished the Upheaval series, but there was a third alicorn who controlled the "outside" of the bubble Equestria was in and fought to protect the utopia Celestia wanted. He is a bit cynical and pissy. He was earth, I think.

2132032
Ah, that would be the Earth King. The story starts as a HiE for a few chapters, but then turns to a fellowship questing against him because the God of Earth is not renewing the planet. No food is grown, no gems are delved until he gets his revenge.

It would be Skyla, the alicorn of being cooed over how cute she is. I thought we already knew this?

alicorn of the seasons maybe

2131961 Apparently they're already releasing a Princess Skyla

2131961
How about something different like....I don't know....a guy alicorn?

2131961 I vote for Princess Vinyl Scratch. She would reign over music, inspiration and of course, da wubs. Her domain would be a massive night club kingdom that did nothing but take in ponies to produce Equestria's finest forms of art; whether it is music, painting, poetry, etc etc. Then through out her long and prosperous rule, a new pony caste called The Muse would be born. These ponies would have the ability to inspire others in whatever field of creativity they work in.

2131961 Whatever it is, it will allow canon fivesome princest.

2132481 Princess Octavia, in my opinion.
A princess needs class, and Octavia has way more than Vinyl.

2131961 who the hell is nyx?

2131961 Well probably Princess Skyla. Or (though I seriously doubt it) that Fluttershy toy with her as an alicorn is foreshadowing what will happen to her.

2131961
Rainbow Dash, Princess of Awesome

2132481 that's actually legit. I wouldn't mind seeing that at all.

OT: If not one of the mane 6, then it'd be Skyla, if of course she turns out to be more than just another toy only alicorn. My guess would be she'd be Cadance and Shining's kid? Who knows.

Princess Winterlight, Goddess of the everafter.