It doesn't need to be another currency. You could have a lower security blockchain tethered to the main one, with higher capacity and lower fees but still carrying Bitcoin transactions. In general those are called sidechains, and there is a particular proposal right now called drivechain that might be worth a second look.
There are several other ways of transacting in Bitcoin outside the main replicated-everywhere-forever blockchain, Lightning perhaps being the most well known (with the defining feature of instant transactions).
Most of these systems are hard to implement without unmalleable txids, so interest will likely pick up now that we have segwit on the main chain. Those systems never left the idea stage while transactions were free but there's significantly more interest now. There are lots of interesting developments going on.
There are several other ways of transacting in Bitcoin outside the main replicated-everywhere-forever blockchain, Lightning perhaps being the most well known (with the defining feature of instant transactions).
Most of these systems are hard to implement without unmalleable txids, so interest will likely pick up now that we have segwit on the main chain. Those systems never left the idea stage while transactions were free but there's significantly more interest now. There are lots of interesting developments going on.