He was secretly an insider all along
.
My take on it is that you will fight them separately but here's the catch, similarly how
@Storm described it, you will fight most of them "on the same battle" but actually you will only fight with 3 of them (the ones shown in the trailer) with the others only being summoned as extension of their attacks (think of it as
Yuna in Dissidia 012)
They could make them Chapter 15 exclusive bosses.
Correcting myself here, but It looks like you fight them in the same room that gets progressively destroyed as you fight more of them.
And its more likely all of this is an extension of chapter 14 with some post game content. If you look closely at the trailer, all of these bosses are in the 40s-50s (except for Omega). ALSO they are all part of the Cure of Insomnia questline, which is the final questline in the game. This whole part in chapter 14 just got massively expanded.
Judging by location, it seems that instead of just fighting Ifrit -> Ardyn, they added many other bossfights and cutscenes and areas to this section of the game. Ardyn putting up the shield around the Citidel makes it seem like you have to fight a few bosses to take it down.
Speculating, but I think Cerberus is placed outside of the Citidel for this exact reason, along with potentially other monsters to fight (Including the ones that were in the original dungeon). Fighting Ifrit to get into the Citidel will probably be similar. But, then it seems that you now have to actually go up the Citidel slowly and fight these Rulers of Yore before facing Ardyn.
The reason why I think this is because Cerberus is Level 45 in the trailer, then 2 of the Rulers are levels 48 and 53 respectively. And there are also new cutscenes revolving around these bosses as well. It is also likely they bumped up Ardyn's level or even revamped his fight as a result.
Omega is level 119 in that trailer, so he's totally post game.