Make the handle X-shaped.  Put the drivers in the corners.  

Give it a couple of hitch points, too, to minimize the amount of sway the cargo can have.    Make it a split-X (double-Y? ) with the hang points under the junctions, and the direction of travel is the long axis.  You might get a little side-to-side sway, but that should minimize the back-and-forth in the direction of travel.

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My cordless Dyson hand vac has what appears at first blush to be a really short battery life.  I have yet to run the battery out in operation.   The trick is that the battery is rated in WORKING time.   If you only pull the trigger when you are actively cleaning, the batteries last much longer.  The same applies to this:  Have the lifter programmed to settle and put the box down every time you stop, and you can really extend battery life.

A low-power laser so it can follow terrain and climb stairs. Doesn't need much more than a 

laser pointer or something similar.  Visible or not, depending.

Comes in various sizes, with a variety of bags and platforms that can be snap-attached.

Bluetooth-analog, so you can mesh several of them together for larger loads/longer run time. 

Maybe a leash with a dead-sophont connector on the lifter, so that if the leash disconnects from the lifter, it automatically settles.  Or possibly just automatically stops, and holds position.

Add a GPS analog, so it can report speed and position.   :)

Elevation should be adjustable, but I can see some sort of hard limit to prevent it from obstructing traffic.  You could limit it to 10 cm after load stops increasing, or whatever, but that's easy to defeat by putting the load on a platform, chair, counter, table, whatever. 

It needs some way to determine elevation (height over floor, not absolute elevation.).  If I take it up in an elevator, I expect it to resume flight height when the elevator stops.

Just the thing for hauling a big box of cat litter, or water softener salt, in from the garage.  :)

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