Increased Mob Difficulty Below 0

Since the expansion to the bottom of the world is supposed to be a challenge, perhaps we could make mobs more difficult to defeat there, too? For example, zombies and skeletons could spawn with armor more often, spiders could spawn as jockeys and invisible more often, and maybe even increasing mob cluster spawn size? (Mobs spawn in clumps of 1-4, depending on the mob type.)