It looks like the Leaf might be a good candidate for a speaker transplant like the one I did in my MINI...
The MINI had mid-range/woofers in the lower doors, tweeters in the upper doors, and full range 6" x 9"s near the back seat arm rests. All the speakers were very cheap paper-coned speakers with tiny magnets. The stock head unit was very custom to the car, and it would have been difficult to integrate the steering-wheel controls with an after market head unit. So, I kept it.
I replaced the speakers in the doors with better quality, and more efficient, drivers. Those "separates" came with their own primitive crossover. In the back, I replaced the existing 6x9s with a pair that had the best bass extension I could find. I gave them their own ~200W amplifier, and rolled them off at 80Hz or so. So essentially, the rear speakers began acting something like a sub-woofer. (I also removed the back seat, so no worries about what people trapped back there would experience. That's not something I'll probably want to do to my Leaf.)
In the end, it all still looks factory stock, but sounds much better.
You will probably want to replace the plastic spacer doohickies that the front speakers are mounted to with custom MDF spacers. Besides removing some potential resonance, that allows you place the speaker just behind the existing grill, and maximize the available depth for the speaker magnet. (It also gives you some flexibility on size, since the speaker mounting screws don't have to match the existing holes. One set of screws attaches the spacer to the car, another set attaches the speaker to the spacer.)