As I stated earlier: In TW2, the Raven armor and endgame swords gave you a little edge 'till you hit the mid/end part of Chapter 1, if you upgraded them properly... I don't really see why that wouldn't be possible to in Wild Hunt... They could very well make them viable through the prologue, or even longer if you upgrade them too... not every armor/weapon, of course, but the most interesting/legendary ones.
...as far as the dagger and Melitele's Heart go: Geralt could get them later in the story, if they become quest relevant. Maybe he stashed them with Triss?
This definitly would have been possible, but the items you mention do look quite end-game, then after the prologe you would again equip gear that does look much less legendary but would be better regarding stats.
Again I don't think it is impossible, but there are gonna be similiar items for sure ( visually) and just keeping them for the later game progressions seems for me quite the better solution.