At least the medium/lush biomes could have four seasons (spring, summer, fall and winter). The cold (snowy) or hot (desert, jungle) biomes could be excluded from this.
What do seasons change? The most visible change is the vegetation, for example colored tree foliage in autumn or blossoms in spring time. In winter the precipitation would be snow and not rain. And it could also have some impact on crop growth, like slow growth in winter. This would add some difficulty to the game. Mob spawning could be affected by this too.
There are already moon phases in Minecraft. Maybe the seasons could be tied to this. One entire moon circle could be one season. I think it would make the game more diverse and enjoyable.