Populated suburbs and ultralight aircraft help?

The only way you can operate an ultralight over a densely populated area is if you get a "waiver" from the FAA. Generally speaking, only one-time waivers are available for things like special events. Unless you were involved in law enforcement, wildlife work, or something similar they won't give you a waiver for unlimited flights. The only reasonable way around the restriction it is to get the aircraft certified in the experimental category and get a sport pilot or private pilots certificate to fly it that way. That probably won't happen with a jet pack in your lifetime. Another problem is that except for takeoff and landing,you would have to abide by the 1,500 foot minimum altitude restriction over a densely populated area, which means that in many cities with Cl B or C airspace you would be required to have a transponder plus communication and navigation equipment that most ultralights and a jet pack certainly couldn't accommodate. If you're seriously thinking "jet pack", you're dreaming. You won't be able to afford it , the maximum flight duration is only a few minutes at best, and the limited performance wouldn't allow you to get to a minimum legal altitude.