The terminal has a modular design and could be expanded south across what is now the taxi rank. I think the real issue would be the CTA. This can't be expanded and any additional expansion to the terminal just shrinks the available space for the road layout, bus station and parking.
A plan was mooted by the airport many years ago to move the landside facilities to a new building on the other side of the tunnel leaving the existing terminal as pure airside. Going back in time even further there was another plan to build a pure landside terminal over the railway line. It would be hugely expensive and some sort of driverless train would be needed to join both buildings.
Of course there always remains the option to expand south of the runway with terminal two but both these options would be years away.
