In Wind and Wave, the main theme is ecology. There is a new organization, which is a foundation whose goal is to preserve the ecosystem and habitat of Pokémon. Is the same organization that later on gave funds to develop the PokéVita program in Pokopia.

Main legendaries are said to bring extreme weather conditions, similar to Kyogre and Groudon, but more related to strong winds, hurricanes, typhoons, and tsunamis, rather than the expansion of land and sea.

The whole gimmick indeed involves weather, and weather moves become more important than before.

New forms of existing pokemon changing depending on weather. There will be a special weather condition involving electricity (rain vs thunderstorm differentiation).

Castform is back and gets new forms. Jolteon and Sylveon counterparts as well.

Hints about this are all over Pokopia. Winds and waves legendaries are also partially involved in what happened to the world here, but in an alternate universe where everything went wrong.