If we go by your definition of "cut content" then I can assure you that there are probably hundreds if not thousands of things that are cut from the game somewhere along the development cycle. And I'm not even talking about something being "cut" and then reappearing in the form of DLC, I'm talking about cut and will never make it back to the game.
Your definition isn't only one from an extremely cynical point of view, it's also simply a very irrational one. Having only Noctis controllable is a decision made out game mechanic as well as plot reasons. When you make it so that the other party members are controllable as well, that would mean you need to tweak each and every encounter in a way that it makes sense story-wise that these party members also stand toe to toe against these powerful foes. As it is right now from a story standpoint, the other party members are an aid to Noctis. Important aid, but not special like Noctis is. Think heritage and the special powers that come along with Noctis, think the summoning of the Astrals, etc.
If you make the other party members controllable in the main story, you need to customize every single encounter in such a way that it makes sense plot-wise that the gamer chooses to for example use Prompto as his main character to take out all these powerful foes without the special powers that Noctis has. You can then argue that you can still use Noctis' special powers by giving Noctis' commands, but then you'll end up with the main character of the game as well as the one with these insane powers being nothing more than a sidekick.
You can then argue that making the party members controllable could be a timed event. But that's already the case, to a lesser extent. You get to choose their special abilities when they're up and you get to execute those from their point of view. The moment that you add the option of being able to control them for say a minute, or perhaps 5 minutes at a time, you need to explain that and make it believable. Why can you control them but only for such a limited time? That's a very jarring mechanic and one that is much better ignored all together by making it clear from the get-go that you're Noctis, you're controlling Noctis and the party members are your aid.
Now, in the DLC, they have a wide range of possibilities of incorporating the controlling of the party members. Since we know nothing yet, it's all left to speculation. They can easily make these DLC a single-character experience in which you only control Prompto and don't even have the other characters around you. Think of the anime for example in which we see these characters in a context in which they're not surrounded by the others. You can make a short experience in which we see the few hours or so leading up to these characters meeting up with the others. Heck, you can even make a short experience in which these characters do their own thing long after the main story line of the show. There's countless of possibilities there.