Magicka Nightblade in particular suffers from a major issue in the PvE endgame. This issue is that when it's not the best DPS it disappears. Nightblade doesn't provide anything to assist it's magicka friends that can't be found elsewhere. Here's a couple skills I'd change to help alleviate this.
Soul Harvest:
Remove the 20% increase of damage you deal, and instead add a new debuff that makes the target take 6-8% more Magic damage for it's duration. Majority of NB's skills are Magic Damage. With the prominence of Orb as well in the meta as a magic skill, this could leave 1 Magblade to being more desired in any comp for this buff Making it akin to Engulfing Flames but for the Magic Damage type.
Veil of Blades:
|Make this function similar to the Eye of the Storm AoE morph of the Destruction ultimate, sticking to the nightblade to have a utility/damage option. May not be the best solution to make this skill the best, but could be a way to give them more flavor.