The Meaning of Life, the Universe, and Everything.
In Vanilla Minecraft, I've always found the lack of creature variation to be somewhat confusing. Some mobs, such as villagers, rabbits, even zombies; have some different variations based on where they're spawned- whereas others, such as pigs and cows- all look identical. There's a lot of resource packs that fix this by adding multiple cosmetic variations for common mobs; but what if these actually had an effect on gameplay? This is mostly just a concept, but I think it would be really interesting for people who enjoy breeding mechanics to create new and useful creatures- (Such as with dyed sheep.)
-More variations for wolves/dogs, which currently all look identical. Perhaps Dark Wolves for Taiga or roofed forests, and Arctic Wolves, found in rare cold biomes. Dark wolves would be tougher but harder to tame, whereas arctic wolves would be fast and have a chance of inflicting slowness on enemies. In addition, I know the Quark mod adds a fantastic Fire Hound to the game- a netherworld version of the wolf whose attacks set things on fire... but that can be rewarding to tame, if you're daring enough. Perhaps different breeding combinations would result in even more special variations?
- More variations for cows! We already have red and (ultra-rare) brown mooshrooms, and, (If my hopes are correct), a Moobloom following the Minecraft Live vote- but it would be neat if there were even more biome specific variations. Consider Mooglooms, a Nether version of the Cow unique to Soul-sand plains, that would be hostile and creepy, but grow netherwart on its back; Moolooms, the result of breeding a cow with a sheep, that would produce wool and milk, or even Moobooms, a cross between a creeper and a cow, that exists only because it rhymes, and I couldn't think of much else. On something of a side note, I also like the Tortoises from Quark- creatures found deep underground tha tgrow mineral types on their back. Maybe this could also be incorporated? In addition, it would be cool if there were different versions of the Moobloom for different flower types, the type of which would vary based on how it's bred.
-We've got biome specific villagers- but it would be interesting if the Iron Golems also had some sort of biome-specific cosmetic variants based on whether they were found in cold, dry, or ordinary biomes.
-Possible variations of the parrots that would essentially add birds to other biomes?
-And, last but not least, pigs and chickens. I can't think of any particular uses off the top of my head, but more biome-specific color variations would be neat.
-Biome specific variations for creeper colors that, matching the grass color of said biome, would make them even more irritatingly difficult to spot. In this way, you'd get albino creepers in the arctic, golden creepers in the Savannah, grey creepers underground, and so on and so forth.
-In addition to Husks and Drowneds, it would be interesting to see not only more cosmetic variation for zombies, but also more biome specific ones, that, in addition to suggesting what loot you might obtain from them, might also give some clue as to their deaths. (Different textures for woodland, mountain, and perhaps even mushroom zombies.) In this way, the relatively simple system that determines what zombies are equipped with might be altered to include Miner Zombies, who might have pickaxes and helmets, and be able to (occasionally) break blocks, rare Warrior Zombies who would have armor and weapons, a high chance of dropping said weapons, and a chance of having arrows sticking out of them, and even rarer Arcanist Zombies who would wear rare enchanted gold armor, and have a chance of throwing one or two types of splash potions. All this might help to add a bit more variety to the mindless hordes of brain-eating corpses that pursue you through the night. The Rotten Creatures mod does a fantastic job with biome-specific zombie types, and definitely spices things up in regards to the undead.
-Spiders! Everyone loves spiders. Some interesting new takes, in addition to normal and cave spiders, might be, (Firstly), if cave spiders actually generated in caves and not just mineshafts, and (secondly), if there were more types of spiders to deal with, such as Woodland Spiders, a brown variant that could spawn occasionally in forests at any time of day, but would only be hostile if attacked; Weaver Spiders, a paler variant found in abandoned structures, that would be able to shoot cobwebs in order to slow down prey; Jungle Spiders that occur in unusual colors, preffering to... possibly... hurl cocoa-bean projectiles from the treetops? ...And Warped Spiders, an unpleasant addition to the Nether that might be warded off with crimson fungi, and teleport to a random place a short distance away when injured.
-Slimes- instead of just run of the mill green slimes and flaming magma cubes, perhaps; Blue Slimes, a sort of half frozen magma-cube variant found in cold biomes, which might drop some sort of frost variant of magma cream that could be used to create frostwalker affects, and, while slower, would inflict paralyzation on whatever it hits; perhaps these could somehow be bred to create other colors of Slime? ...Because so many people just love slime?
...These are all just vague ideas. I think a mod that added mob variation with more affects on game play, while retaining a vanilla style, would be very interesting! More variation on existing content, is, (In my opinion), almost always a good thing.