Piggybacking on this to say these things should be trivial:

• Electron Muon String microscopes -- Demons should be useful for providing extremely precise measurements.
• Tricorder -- Why bother with MRI/blood tests/xray/etc when you can have medical bindings for djinni?
• Particle collider++ -- it should be pretty much possible to reverse engineer physics.
• Instadeath -- "Apply 20 newtons of force to the brainstem of the person I am looking at if and only if I bite the inside of my left cheek while pinching my right finger, OR, if I say avadakedavra."
• Security systems, cameras, screens, holodecks: As /u/clawclawbite said, control over photons means displays and illusions. The security aspects of this-- and the fact that djinni can cancel the workings of other djinni-- mean that countermeasures would be a pretty big deal too. Important persons would likely have privacy algorithms, something along the lines of "block all light and sound for 60 minutes or until I say cancel." The fact that djinn can cancel each other out ALSO means that attrition can be a viable strategy-- sometimes throwing a lot of djinn at a problem really might be the solution.
• Chemical synthesis -- easy even if you can't modify molecules directly (and we might be able to even do that.) You might even be able to make a Wirehead Gun effect, where you flood someone's brain with dopamine (again, you could tie that to a totally non-obvious trigger.)

Less feasible, still worth researching:

• Invisibility -- As seen here, invisibility isn't exactly trivial even with magic. I think the djinn could do it, though.
• Quantum scanning -- Is it possible to take a snapshot of a brain's state? Can the djinn keep the snapshot in memory?
• State vector forking -- Is the djinn capable of building a human from scratch, given all the necessary raw materials? Could it do so from a quantum scan? Would a perfect clone of a person also be able to summon a djinn?
• Simulation -- could the djinn simulate a person that has been scanned? Would that person be conscious in the djinn-simulated environment? Could that person's perception be accelerated arbitrarily? Can a person-state that has had the simulation operation performed on it be embodied afterwards? (Allows for virtual lairs, virtual kidnapping, and time 'pausing.')
• Teleportation -- We know c is still a hard limit, but can djinn move at c? If they can, it could enable lightspeed travel: a practitioner could have the djinn perform a destructive scan, seemingly causing the practitioner to diappear. The djinn would then travel to its destination and rebuild the practitioner with materials from the destination.

My vote for in-universe character is Dr. Otto Gunther Octavius. He's a transhuman villain with a doctorate; I'd love to see /u/EliezerYudkowsky's take.