Worst case scenario for this game would be if Clive is the only playable character, and summons will only appear in the story, but can’t be used in battle (other than for some special attacks, as shown in the trailer).
Unfortunately, that seems pretty plausible from what we know so far. If summons (‘Eikons’) are linked to individual people (‘Dominants’), who are able to transform into them or summon them, then I don’t see how we can use summons ourselves unless a Dominant joins the party. That’s possible of course, but that would basically mean that only certain characters can summon Eikons, and only one per character. But if that was the case, why is Clive in the trailer shown to have powers from multiple Eikons (Phoenix and Titan, at least)? Wouldn’t that be redundant?
It seems to me that Clive gains those powers either by killing Dominants (in the trailer he’s part of a group that hunts Shiva’s Dominant), or if a Dominant grants him part of their power voluntarily (probably how he obtained some of the Phoenix’ power from his little brother). The only way around that is if we can summon at least a lesser form of those Eikons, like a spectral or spirit form or something, while a ’full’ summon can only be done by Dominants in the story.
And I really hope we get playable party members, but with the game’s apparent focus on a character action style combat system, it’s hard to see how that’s possible. They would basically have to design unique battle styles for each party member, each of which would have to be balanced individually in regard to the game’s difficulty. I’m pretty sure that’s also the reason why Noctis is the only playable character in FF 15.