If the I-MiEV continues in production, I doubt there could be any changes to the vehicle. At the current selling price I am sure there is little to no profit in selling it. That also leaves no money for engineering or retooling for a replacement model. It's possible that the existing parts inventory to build them is high enough due to low sales volume that they can do a small production run every couple of years until parts dry up and that will be the end of the line. If Mitsubishi originally forecast sales to be far higher than actual numbers, they would have ordered major components from various suppliers in far greater quantities than needed for actual production and therefore have a load of inventory of unusable parts. Couple that with the fact that the I-MiEV doesn't seem to break down a lot and a small crash will usually total the vehicle, the replacement parts business isn't very good. So, if you still have the assembly line and tooling, still build the KEI class gasoline car in the same body style, its a great way to get some value other than scrap for your excess inventory. The I-MiEV is a lot more expensive to build than, for example, a off road side by side utility vehicle (i.e. Polaris, Artic Cat, etc.) yet is similarly priced. If you go with Polaris' offerings in electric side by sides and motorcycles, the I-MiEV is far cheaper. With current low gas prices, low demand and internal Mitsubishi corporate problems, it doesn't take a rocket scientist to figure out how this will end.
Nissan and Mitsubishi have worked together in the past and re-badged various models of each other's vehicles to fill gaps in their respective vehicle lineup. If there is a Mitsubishi electric vehicle future it will most likely be a Nissan powertrain in a Mitsubishi specific vehicle or a re-badging of an existing vehicle to keep the dealers happy. Volume lowers component unit price so anything that can be sourced from a higher volume vehicle will lower the ultimate cost of the lower volume vehicle.
As to battery voltages. The LEAF and I-MiEV batteries are different in design, form factor and chemistry. I don't know if Nissan will get (or want) a stake in the GP/Mitsubishi battery alliance as Nissan already has an in house battery design and manufacturing capability. A future I-MiEV replacement vehicle would most likely use a Nissan battery. Although GP builds similar cells to the ones in the I-MiEV for other uses such as stationary backup, rail use and the Boeing "Dreamliner", the cells in the I-Miev along with the module and BMS controller appear to be I-MiEV specific.