Interesting ideas Sky. Maybe you should be a writer too!
I had an idea a few days ago I was kicking around for the last episode, but I never got around to typing it up.
Essentially Part 1 proceeds as normal until the ponies are all dressed up and they reach the gates of Canterlot. They line up behind some other ponies who are waiting to get in where a guard at the gate calls off names and checks them off as they get in (all of them minor characters from previous episodes).
Once it's the Ponies' turn Luna interrupts the Guard when she shows up in her grown (and well dressed) form to 'greet the guests, in accordance with a request from her sister'. All the ponies are shocked, and as they watch she takes their names and checks for them on the list without any recognition.
Luna: "Let's see... Twilight Sparkle and friends... hmm... it does not look like you are on the list I am afraid... what a pity."
A confrontation ensues, and the guards escort the ponies away. Infuriated that Luna has come back to ruin their big day, each of the ponies in turn suggest and try different ways to get into the castle to speak to the princess but they all fail (Pinkie Pie tries a disguise but it falls off, Rainbow Dash tries to bolt in a window but they are magiked shut, and Twilight tries a spell to make the guards ignore them but it instead turns them into chickens). The whole time Spike has an idea, but he gets ignored until everyone has tried and failed.
Finally at the end Spike says 'Why not just send her a letter?'. They do it, and the Princess lets them in right away. Going up to Celestria's Throne (with Luna right there beside her) they storm on about how Luna took them off the list so they would miss their big day.
It turns out that Celestria actually forgot to put them on because she had been so busy. She apologizes to the ponies and suggests they do the same to Luna. Twilight does, but Luna sticks up her nose and prances away.
Celestria: "Don't worry, I'm sure she'll come around eventually. In the meantime you are all missing the gala..."
The episode ends as Part 1 of the 'Best Night Ever', with Part 2 being almost identical to the one they aired last week.
