Alpha Centauri. With Brian Reynolds as a designer.
And no, Civ:BE doesn't count as a sequel, it doesn't even have voxelized terrain with stuff like rain shadows, etc. Not to mention a great story. It's just mind-blowing that SMAC had features that current versions of Civ still lack after all these years.
I would rather see someone do to Civ what Cities Skylines did to SimCity.