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Here's my thinking on a BMS. Each cell should be able to be switched out completely. So, for instance, there are 2 MOSFETS on each cell, one to enable output (Connect from + to out) and the other to disable output and short the cell (Connect from - to out). This will ensure that, upon error or failure, the entire pack doesn't die, the voltage just drops by one cell's worth. This can continue until the entire pack (or at least the majority of it) has dropped out, or a low voltage condition forces a stop. When charging, this means the cell is charged to full and dropped out completely (a signal could be issued to the charger to drop input voltage by one cell's worth) to continue charging the rest of the cells, so every single cell is charged to completely "full".
This would give both top and bottom balancing. It would have the entire pack's entire capacity available for use, and would give a better "ratcheting down" of available power to let you know you're nearing the end of your range, rather than a sudden drop off a cliff.
However, it would mean that series strings would need to be widely paralleled, as the MOSFETs would severely limit the available current. Alternatively, small relays could be used. But no matter the switching system, it's that switch that would be the limiting factor, unfortunately.
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