Ok, I've gotta go to sleep for work tomorrow, so I'll just post my unedited responses to the first three paragraphs you wrote. I am willing to make drastic changes to the character if I need to, but for now I'll just try and get you as much information about the one I have as possible. 

The reasoning behind the assassination thing was that there would be people looking for those who gained these abilities (I figured at least one group in the world would be trying to prevent the spread of knowledge about AMA/would be trying to stop the resurgence of magic), specifically Heaven’s Hand or some other underground organization. 

I figured it would be an OP skill to have, but I also figured the ability could be as much a curse as it is a blessing. I planned on the eventuality of his character dying relatively frequently, allowing for infinite possibilities on who he takes over as well as what situation whatever he takes over is in. For example: he could take control of someone in jail, or a pet bird trapped in a cage. Or he could take control of a morbidly obese person, a cripple, or an elderly person.

Perma-death was also something I thought over quite a bit. I’ll be honest; I figured that since his character was more of a specialist than an actual fighter, it wouldn’t matter as much if he died. Ways of him dying forever would involve his conscious mind not being able to swap to another body, someone attacking his consciousness through other forms of telepathy, and eventually a steady decline in his ability to find another host. I also thought of the potential of a mind “snapping out” of its dormant state and fighting back against Jack’s presence, occasionally ejecting Jack’s mind. Lastly, the process of taking over someone’s mind and then having them die on him leaves a permanent impression of the thing’s consciousness on Jack. 

(none of Jack's higher level tiers have been thought about in detail yet, so if this stuff that I'm writing seems OP its because I didn't take the time to think about it too much. I planned on the higher tiers actually decreasing the OP nature of Jack's initial ability)

I planned on having the higher tier unlocks mainly be based around him breaking his consciousness into pieces, allowing him to briefly influence someone’s actions, feel someone’s emotions, or make them feel something he creates. The only way he could actually fully take over someone else’s mind (until very late game where one possibility I thought up involved him creating two functioning conscious minds that could act independently) is if he were to fully remove himself from his current host (whether or not the host regains conscious control of his or her mind is based on how low of a tier Jack is) and place himself in a weak willed mind. This would be around tier 4 or 5. Actual mind reading was not something I saw as being possible for this character, as his mind never straight up melds with someone else’s. I wanted the person or animal’s mind to temporarily be suppressed (kind of like when you’re blackout drunk) while Jack is in control.

I'll check back tomorrow, as well as do some more specific work on this character, but I am willing to make a new character if this is too much of a headache for you.