I've been going over this several times looking for information to help deduct what it could be. Tabata said that it wouldn't be a boss fight and he said it was something that FF fans would like.

Emphasis on FF FANS.  Something tells me it has to be something old school based, but it could also be an announcement of some kind...maybe for another game, who knows.

Since he said there wasn't going to be a boss fight at the end I'm thinking the surprise can't be a summon either.  I just can't see how it could be a summon since they're treating this like  a big surprise, and Type 0 itself has summons in it.  But it isn't out of the realm of possibility.  

If I had to guess, everyone including myself is hoping dearly that the surprise is an airship.  Not just any airship either.  A FULLY CONTROLLABLE airship that we'll be able to use to fly to every destination out there.

In all honesty, that needs to happen.  It wouldn't be so much a surprise either, because it's more of a necessity in a FF game that should have always been.

One thing he has said though is that he and SE don't want to put in anything just to please the fans.  It would have to make sense with the world they created according to them.  However, controllable airships seem very possible in the world.

I noticed in the TGS 2014 trailer that there was an airship of some kind in the sky while Noctis was fighting with Prompto.  It's where Prompto I believe jumps over Noctis in the trailer and then Noctis stabs a soldier.  The important thing to note about this is that it was happening in real time, and in the open world setting of FFXV. 

The only three things I think it could possibly be are:

1. An Airship

2. A Summon..maybe

3. A game announcement.  Maybe Kitase has already jumped the gun and is about to announce he's working on a remake of a certain popular  game.  He did decide to leave the producer position to go work on something I believe.

Edit: Ah yes, I meant to end with asking: So what do my fellow VGCharters think the surprise could be?

Release date would be the only thing that would surprise me.

Lightning makes a Cameo and replaces the main character and every other character with herself

Release date or KH3 trailer.

If Toriyama was directing it, sure.

Ah yes,  I suppose that could be it.

Wii U exclusive.

Release date or Teaser for another game.

FF15 Now being developed for PS5 and XB2, or release date 2020. :p

I would guess an actual release date, the airship thing or another game of some kind (probably remake).

He specifically said something that Final Fantasy fans will like. It really can't be about KH or another game.
I really have no idea, but his words make me think that it is something nostalgic..