It wasn't clear to me why adding a common base stage was particularly an improvement - it does increase the output impedance, but the current gain is slightly less than unity.It doesn't. Your drawing says, "improved stability with load voltage variation", not, "perfect stability with temperature variation".
The first improvement I'd make, is a stabilised voltage reference for the base of the bottom transistor - if temperature stability is important, its possible to incorporate compensation into the voltage reference.