Size: I'd suggest several standard sizes in multiples of 15cm (6 inches). For example 30cm cube. Connector: Should be recessed so it doesn't extend beyond size envelope. Voltage: Probably would need several. Connector should be unique to voltage. Multiple of 4.44 volts (NiMH x Li-Ion voltages).

Lack of standards holds things back when companies need to make a very large investment and need to know over what timeframe it will work. These standards could be in place for a long time. The "D" cell battery standard is over a hundred years old.