Hideo Kojima posted a list on his Twitter of the themes of the Metal Gear Solid games, in order of release.
— HIDEO_KOJIMA (@HIDEO_KOJIMA_EN) September 9, 2012
“what we inherit to next generation” was the theme of MGS. MGS1 was Gene. MGS2 was Meme(cultural gene), MGS3 was Scene(era), MGS4 was Sense(intention), and PW was Peace.
Kojima also summarized: MGS1 is about control of genes, MGS2 about control if information in a digital society, MGS3 about the Cold War in the 1960s, MGS4 battlefield control in the near future, and MGSPW nuclear deterrence in the 1970s. He also wrote something about the different types of enemies in the series:
Enemies in MGS. We appointed special force in active service (elite) in MGS1, American comic type of superhuman in MGS2, legendary special force (mysterious men) in MGS3, monstrous victims of war (monster) in MGS4, and unmanned AI weapons (huge mech) in PW.
Unfortunately he didn’t provide any new information regarding Ground Zeroes.
Source: Hideo Kojima’s Twitter