New Powers...
Every great sequal introduces new moves for the player. In the gadgets department, i would add and EMP device that could disable security devices. For the supurnatural powers, i would add a 'hypnotise' spell, which would be like an AoE version of the sleeper darts (it should use a lot of mana, obviously).
Waking Enemies...
To reballence the game, and add a new challenge, enemies can wake an ally whom you have knocked out. If they see a sleeping ally, they will run over to them, and revive them with smelling salts, or simply shake them awake. The woken enemy will then enter the alerted state.
Staging a 'nap'...
You can however counter the waking enemies by staging a nap. Rather than leaving a KOed enemy on the floor, place them in the nearest chair, bed, or sofa. If an ally of their sees them in this state, they will simply belive they are napping on the job, and will wake them in a calmer way. When woken the emeny will dismiss their encounter with you as a dream, and will not raise any alarms.
What are your thoughts?