Going by the dates on the press releases, the original XP-G was announced September 2009, and the XP-G2 was July 2012. The interval was substantially larger for the XP-E/XP-E2, and less for the XM-L/XM-L2.
I think the answer you came here looking for your question "What comes after XP-G2" was "XP-G3". However, I will submit that the proper answer is "Lights that take advantage of the XP-G2".
Replacing a cool white XP-G2 into a light that formerly had a cool white XP-G will yield only marginal results; however, since the release of the XP-G batteries and manufacturing standards have come much further, and the XP-G2 overdrives well. Take the Armytek Predator line, which rather than just straight replacing the XP-G, they increased the current draw, making even the 4000K neutral/warm substantially brighter than the cool-white XP-G R5 version.