A life lesson HAS to be learned. That's the only way I could see MLP Ending. A lesson has to be learned and it has to effect the viewers. You cannot end a kids show on a whim like that.
Whether it be someone dies, RD becomes an official Wonderbolt, Rarity gets married, Pinkie Pie.. I don't eve know... AppleJack loses the farm.. Twilight is revoked of her powers..
All of these I've listed teach a lesson. Try hard enough you'll get there. Love will find a way. Stay smart and don't take too many chances.. Things might not always work out, enter a quarter to try again.
But if I had to choose.. I would suggest that Twilight is revoked of her powers for not being a successful entity of the Power of Friendship and the show ends on Twilight writing the story of her friends down in a book while they're old.. Granny Smith old, and as the last word is written she takes a look out the window see's Scootaloo as queen and ruler of the universe and then end credits.
BAM!
Scootaloo confirmed best pony.