Opening music courtesy of Harpeth Presbyterian Church, closing Waiting for the Robert E. Lee, courtesy of BanjoHangOut used with permission.

Yes, we may be able to make incremental gains, but they are going to be increasingly more expensive. 
Changing the shape of a battery may increase the available storage/energy, as long as you don’t mind a smartphone the shape of a Campbell’s soup can.
Perhaps the Motorola Razor was the optimal design, after all.
No battery is a forever device! Leave that thinking to the USPS. The goal of a forever device is illusionary.