Possibilities for the end of the demo:

It is a big boss, those liars!

It is Big Boss. SNAAAAAAAAAKE!

A release date.

The demo ends with you walking into an abandoned looking town, fog rolling in all around you. The music starts to get a bit creepy, and the radio you carry with you starts to get static-y. You move your flashlight to the distance where you hear a sound, the static getting louder. A large shadow appears in the fog before you. The the words pop up on the screen: Silent Hil(SPOILER)da Garde! The fog clears and there's an airship! Oglop Cid hops down from the ship! The fog was Mist all along! Oglop Cid then proceeds to tell us "Airships are back!"

A code appears that will unlock SOMETHING in the final game, but they don't say what. That code actually is a Hypello skin which turns literally every character in the game into Hypello. Ride ze Shoopuff? Shoopufs waiting. All aboards!

Lightning.

Lightning, but she has bigger boobs.

A download code for FF I.

The words Mistwalker appear on the screen. This game has actually been secretly codeveloped with Sakaguchi for 10 years! Nobuo is also back.

A reveal that it is only coming to Japan. Us westerners don't like Japanese games.

A reveal that it is only coming out on mobile. It will be waiting for years for mobile technology to catch up to their vision.

Triple Triad is back.

A very realistic looking Mukki falls from the sky to reveal that FF VII is getting remade.

Mukki is actually revealed to be the main character.

The words "In Development" flash across the screen. In smaller letters beneath "More info will come soon. Stay excited!"

Tabata's face appears on the screen. He starts excitedly stating that the moment has finally come. The moment we've all been waiting for. We've been waiting for years for this announcement, to know when the game is finally coming out, to have a release date. And then he whispers,(SPOILER) "April Fools." The Tidus laugh starts playing. The demo was released on April 1st, which you just now realized. This was an elaborate April Fools joke 10 years in the making.