Since we're considering new DLC's, I think the one which would give new players gunnery encyclopedia and ship handling books for some additional payment would be really useful. It would limit the gap between new and old players a bit. In the same time it wouldn't be "pay 2 win", since you can get those books through in-game actions.
5 ring book is less critical. Two mentioned above, especially ship handling, really make a difference in PvP.