Kratos would be easy to expect, but he's not all that interesting. In fact, the rest of the playable cast is confirmed to show up, so they're not special. The mana spirts are likely to show up again.
The PS2 Symphonia received an extra Kratos+Yuan+Mithos hidden boss, and I want to see a similar boss-team included.
A difficult tiny-boss like Abyssion would be welcomed, but a difficult big-monster should be included as well. And a cash-cow enemy like the mine dragon should be a no-brainer.
A boss-team like Sylph, that was actually coordinated, would make good eye-candy.
But since the new aspect is animal-collecting, there will probably be brand-new animal-spirit bosses to surprise us.