Not too long from now all primary altimetry will be GPS derived and barometric altimeter setting will be retained purely as back-up. On commercial aircraft local QNH will be auto-uploaded by data link and inputted to back-up barometric altimeters, both to confirm GPS altitude and to give redundancy. For GA pilots the option will be available to adjust indicated altitude to height for local flying.
It could be that the term 'flight level' is retained, meaning - as now - altimeter indication (in feet) divided by 100 and the word 'feet' will become redundant, removing the complications of parallel Imperial/metric systems. Perhaps for low viz ops decimal FLs for Decision Altitudes could be specified.