The Shield Biome would be largely similar to the Canadian Shield. There would be several variants:
Snowy Tundra Shield
Shield Biomes would have an extremely high concentration of generally shallow lakes, ranging from small to massive in size. Many would also be connected by tiny rivers.
Additionally, Shields would be extremely flat (not as flat as Superflat, but a LOT flatter than Plains).
Finally, instead of normal dirt, there would be a new variant: permafrost. Top layers of permafrost would grow either moss (would look similar to the color of grass in Extreme Hills) or podzol. Podzol would generate in the heavily forested zones. Additionally, there would be numerous rock outcroppings spread around.
Permafrost would take as long to mine as iron ore, and require at least a stone pickaxe to mine. Smelting permafrost would give you normal dirt.
The Tundra Shield would be similar to the normal Shield, but not have any trees in sight. Podzol permafrost never generates.
The Snowy Shield would be similar to normal Shields, but be covered in snow, and all lakes would be iced over.
Snowy Tundra Shields would be like the Tundra Shields, but snowy and icy, as the name suggests. Again, Podzol permafrost never generates.
Finally, a major point: If any ordinary dirt is rained on in a Shield biome, it would turn to permafrost, similar to how farmland becomes wet when it rains.
Considering the nature of a shield biome, would it not make more sense for it to snow? And I like the idea of adding a new block to the game, in this situation. Does permafrost have any properties? Soul sand gives slowness, for example. Is there anything unique about permafrost? Maybe it transforms nearby water blocks into ice?
And what mobs, if any, spawn in this biome (aside from monsters due to light values)