Right now the only 2 really meaningful aspects of his kit are his spellshield and the subsequent buff that comes with it and the Q trail. The second one is sort of negated by his ultimate because you dive right onto your target, but then again you can't really make it harder to hit or w/e because then his ganks pre-6 would be awful.
TBH attack speed as a buff for his spellshield feels kind of weird. Maybe entirely remove the passive AS he gains and put that power somewhere else on his kit.
Ex : Blocking a spell creates a burst of energy around Nocturne that deals damage OR deals damage back to the person who cast this spell.
Make his E create a small nightmarish orb that tethers to all nearby enemies, creating the same effect the current E has. This would need a lot of tuning, of course, but it would be much cooler and would give some sort of pseudo-tankyness through AoE CC to Nocturne as he dives in. This would also be another thing that sets him apart from Vi, who is also defined by her very long range, potent level 6 ganks. She has more single target damage and lockdown, he's more of an AoE guy.
EDIT: Maybe he could feed of people's fears and gain bonuses like damage reduction depending on how many targets he hit with his E or the affected targets deal less damage to him for a medium period of time.