Maybe each of Final Fantasy XV characters or the playable ones or the party members, play instrument in the bar (which the mini game that I like to see is Final Fantasy character play instrument) and play the classic Final Fantasy soundtrack

If Square Enix could, they should create the water area and underwater area explorable. Like going underwater with diving suits. If not, just go directly underwater and fight enemies there.
If I'm not mistaken, in the first or second gameplay trailer (after the name changed to Final Fantasy XV), Noctis come from underwater right the before trailer about to finish. But I don't think both the water and the underwater area are fully explorable. Or is it already fully explorable?

But this is my opinion