Saw an ad for "range extender" for the Leaf that was just a battery pack fitted in the hatch area.
I see a lot of problems with that, weight and safety being the big ones. Aside from putting all that weight at or behind the rear axle, there is no separation from the passenger compartment in a bad crash.
That is before we get into the integrating with the cars electrics issue.
a new 40Kw pack and realistic expectations on what the car can and cannot do sounds better to me.
I'd love to be able to have my Leaf as my only car, It is comfortable and quiet, but I went in with my eyes wide open, and know its limitations, and the fact those will grow as it continues to age. If I have enough time to save enough before range becomes a real issue, then a new replacement battery with more Kw than the original may be a possibility.