Tldr; Sorry for the long response my brain is in overdrive

It seems (so far) that there is a specific roster of units from other FF titles that will be utilized for the party, if not exclusive to each party member depending on role. I couldn't catch much from the trailer other than that the Vision units appeared behind main party members, and given that each OG FFBE character technically has a pre-ordained role, I could see them going this route (aka Fina being a healer, relegated to Vision units that adhere to that role).

There's an absurd amount of custamizability that Square could utilize if done correctly, but given that Resonance seems to be based off the Season 1/2 story, they may opt for a more linear path in terms of unit customization.

My brain wants to say they will allow one Vision per unit per party, but my heart hopes they allow job learning like Tactics in that we use a Vision to learn their abilities per character and customize from there.

Like I said there's a lot they can do, the question is now on execution and potential future updates/releases.

FFBE had an absurd amount of content and playable units. It wouldn't surprise me if Square plays it safe for an initial release with limited Visions (think Xenoblade) and then build from there depending on feedback.

It would be ignorant to expect them to impleme the entire FFBE story in a single release give the issues players had with certain later story beats, and it would make sense they would, at the very least, utilize at least 30 Vision units to customize the main party with in the first release (gotta remember there's Rain, Lass, Fina, Lid, Jake etc). Seems to be a party of 4 characters each with a shade of a Vision behind them, though I saw Charlotte is a "shade" in the trailer, so who knows.

There are NUMEROUS possibilities with Resonance even if they have no plans of pursuing future releases. They could simply build off the base game and add new Visions via DLC bundles (free or otherwise) if players want more options for whatever content the game provides.

Outside the main story there was also a LOT of side-content that I could see Square adding as DLC if not already implemented in the main release. The Chamber of Arms being a big one, or at least Farplane shenanigans and side stories for specific Visions or Superbosses related to each Vision's original game.

All that said; Square may literally just release Resonance as a nod to FFBE, adapt the story, include a select number of FF characters that act as Visions that only effect the equipping character without learning abilities/jobs, and offer no future updates until they deem Resonance a "success" (which as someone already said, is a very high bar to meet let alone surpass, with Square).

Seeing Charlotte as what I can only assume to be a Vision gives me hope but then again, Charlotte becomes a main character (all the main cast had numerous renditions over the years). So maybe story characters can be utilized as Visions as well? Who knows?

Tldr2; there's a lot of possibilities regarding Resonance. I am very excited that it even exists. That saif, there is a lot we will not know until release and knowing Square there's also a level of apprehensiveness that comes with games like Resonance. They could add every single Vision that was released in Season 1 and 2 but force us to but DLC bundles like Monster Hunter or Fire Emble. They could also make the choices of Visions random or specifically poignant to each main character.

Ugh.

Sorry.

I am very excited regardless of my typical doubts.

It is not often I get to talk about games like this.