That's just what BMW is doing with its upcoming "i" range, with an i3 at one end and the i8 (star of the recent Mission: Impossible – Ghost Protocol). So what does it have planned to fit in the middle? According to Autobild out of Germany, an i5.
The third model in the range is tipped to come in as a compact MPV in a similar mold as the Toyota Prius V – only probably significantly more expensive. Underpinnings would likely be borrowed from the i3, but stretched out a bit to make extra room for people and stuff. For that matter, we wouldn't be surprised to see an i2 come in as a two-door version of the i3 with the Smart ForTwo in its sights, or an i6 or i7 electric sedan, either.