A few reports and our own sources indicate that the next two models will be the i4 and i5; a smaller coupe and a family focused four-door.
Both models will be based around the carbon fiber reinforced plastic "Life" structure to keep weight in check and use variations of the drivetrains fitted to the i3 and i8.
The i4 will supposedly be based on the i3's architecture, but will nix the extra set of doors and seats to become a two-seater with a sloping roof, wider track and a more sporting demeanor. Power will likely come from a variant of the i3's electric motor, with the same optional range-extending engine mounted out back. BMW wants to have at least one affordable i-branded two-door within the next five to six years, and considering the i8 will likely crest $100,000, the i4 will be a more attainable entry point for the marque.
Less is known about the i5 aside from the fact that it will be a more conventional four-door sedan with a coupe-ish profile and power provided by the same plug-in hybrid drivetrain as the i8. That could mean up to 400 lb-ft of torque when the turbo'd 1.5-liter and electric motor work in unison, creating one incredibly compelling hybrid sports sedan.
However, both models are a ways off. Concept versions should arrive sometime after the i8's launch in 2014, so don't expect either to arrive until late into the decade.