I would rather have it be a 6 episode finale.
Episode 21-22: Celestia and Luna reveal to Twilight that Cosmos, their brother (let's use the old movie plot) is awakening and that she needs to do something to stop him. In the meantime, she fights Grogar and Majesty but fails to stop Cosmos from waking up
Episode 23-24: Cosmos returns and quickly dethrones Celestia and Luna after a long battle. Twilight attempts to fight him but her, Starlight, Trixie, Sunset, Spike, and the main six is defeated and forced to flee.
Episode 25-26: The final showdown, Cosmos is defeated, show ends with a 1000 year old twilight closing a book sighing in memory and looking at the stars happily.