This isn’t really a biome issue, but an “overall” issue, and applies to preciptation. There are plenty of “winter” deserts in real life as well: http://www.mbgnet.net/sets/desert/cold.htm
It would also be cool to have a world with an above-average or below average temperature as well, to be honest…
Biomes could be regional modifiers, sometimes strong enough to override the worldwide trends completely, sometimes not so much.
So we could have:
World Precipitation: Random - Balanced - Wet (1 to 5) - Cold (1 to 5)
World Temperatures: Random - Balanced - Hot (1 to 5) - Cold (1 to 5)
A wet(5) world would rain almost every single day. There might be brief periods of sunlight, but then the sky closes back up again soon after. Rivers should be larger and spawns should be more common. This is biblical flooding level wetness in some ways. It would be amazing if the rivers expanded to actually go into sections of towns and stuff once we get z-levels in, to really make the world feel properly waterlogged. Keeping dry and happy or adapting to the wet would be priorities. The bionic that pulls water from the air would work extremely well here.
A dry(5) world wouldn’t rain at all. Rivers and swamps would essentially dry up over time, and it would be pretty cool if some fields turned and forests were eventually replaced with a “dry” equivalent where things don’t grow, but the desiccated husks remain - these areas would have their own spawn tables. Water would be a lot harder to come by, and finding and maintaining it (or not needing so much of it) would be a priority.
The 1-4 range for each type would obviously be less severe variants.
Hot(5) and Cold(5) wouldn’t expand the range so much as limit it. Cold(5) would mean that even in the height of summer you would have “light” winter conditions, with chances for snow, and during the winter the “coldest possible days” would probably be the most likely days as well. Hot(5) would have most of the summer be as hot as the current extreme of summer heat, and even winter would essentially just be “summer light” - no snow at all.
Wet(5) and Cold(5) would be blizzard conditions basically all winter and fog or snow conditions basically all summer.
Dry(5) and Cold(5) would be like the badlands I linked above - freezing cold but little in the way of weather beyond biting winds.
Wet(5) and Hot(5) would basically be non-stop jungle monsoon conditions.
Dry(5) and Hot(5) would be stereotypical Nevada desert conditions.