Riding The White Horse
Pinkie's table manners could definitely use some attention, and maybe a full-on intervention. Her sugar addiction is right out in the open, and her manic, ADHD-esque antics are probably directly related to her sugar habit.
Then there's the whole "Pinkamena" thing, suggesting Pinkie's outgoing nature is a facade thinly disguising an emotional powderkeg waiting to explode. If anyone is the poster pony for "serious issues", it's Pinkie.
So maybe an episode where Pinkie's sugar addiction leads to her being kicked out of Sugarcube Corner after eating everything edible in the place, and perhaps a few inedible things. Needing a place to stay, she sleeps on Fluttershy's couch, only to eat everything sweet in the house and drive Fluttershy to use "The Stare" to get her to leave. And so on, from one place to another, alienating each friend, running out of chances.
Eventually she winds up living in a back alley on the wrong side of Ponyville, scrounging through trash cans, licking discarded candy wrappers, doing anything to get a taste of that sweet, sweet sugar she craves so badly.
The show could end with Pinkie being checked into a "sugar clinic", intravenously pumped full of NutraSweet™ and eventually back to work behind the counter at Sugarcube Corner sipping a diet coke.
The letter to Princess Celestia would be simply: "Kids, Just Say No To Sugar."
Hmmm. Maybe I should become a professional cartoon script writer.