Well, I thought about this stuff before, but didn't finish the idea. Here are some thoughts that may implement the weather thing.
Lava rain-> Uses the lava dripping particle to simulate a rain of lava. It gives damage to all races, excluding the Dakkar. They are natural to nether biomes.
Ocean storms-> This would create giant storms in ocean biomes, creating big waves that destroy boats.
Weather effects-> It's about what Yendor said. When spring comes, snow melts, but we could give each season a particular effect:
Winter: can have normal snowing days, but also blizzards. All mobs take damage in blizzards, making it a need to have a safe place for livestock during the winter. During this season, crops that are exposed outside have really low rate of harvesting(from 0-1 drop), making it necessary to have supplies from the former season to go through the hard period(crops that are inside buildings suffer no effect during winter).
Spring: the snow from the past winter melts. The trees that had lost their leaves grow them back, having the default green colour, or even special colours for some trees. Animal breeding gets a boost, making recently bred animals grow at a 2x normal rate. Crops go back to their normal harvesting drops as well.
Summer: I don't have many ideas for this, so I would appreciate some
Maybe a dry season or something.
Autumn: Leaves start changing colour and disappearing(if the tree is an evergreen, it won't lose it's leaves). This is the main harvesting season, meant for people to stockpile items for the coming winter, so crops get a drop boost(maybe 1-3 more dropped items).
Each season may take a month to finish it's cycle, or maybe 15 days to make it into minecraft time.
As I will always say, I'm not a programmer, so I don't know how hard it is to make these. I hope the suggestions help with the initial idea.