Do we know if there's levelling, weapon selection and such?
I would imagine that they won't include levels, as that could create a lack of coherence when compared to the main game, and it's also a very short DLC, so I'm guessing having characters and enemies' attributes fixed makes more sense, especially if they're treating this more as an action game (at least compared to the main one).
Considering the way the trailer presents the whole thing, I doubt we'll get to see Iris during present day. I'm guessing we might have a bunch of flashbacks from Gladio's training days. Hopefully featuring Clarus.