I actually like the idea of different golems built of different materials having different functions. Sure, each of them would have different stats, with Diamonds likely being among the strongest non-boss creatures in the game, but they should also have a unique trait to keep say, clay golems from merely being inferior versions of Stone/Iron, and so on. A few ideas, for instance:
Sand golems would "creep" over uneven surfaces rather then jump, rapidly fall to the ground like their respective block when in the air, and "melt" to be able to squeeze through doors, window panels, and fences as well as single block openings. Their main trait would be the ability to engage hostile mobs from within the confines of an otherwise solid structure, perhaps being stored within a piston door and released at will to attack assailants when needed.
Stone golems, while weaker then iron golems by a significant margin (though still at least slightly tougher then the human player), would be able to throw a ranged attack at an arc (to differentiate their attacks from gold golems and dispensers, as well as give them the ability to attack over walls) at hostiles with a slower rate of fire then their weaker snowy counterparts and less knockback, but be able to damage the target. When paired with snow golems, they would serve as the damage-dealers while their snowy counterparts kept enemies at bay.
As for diamond golems, thinking of an action for them other then making them immensity powerful is proving to be slightly difficult. Perhaps allowing them to break through obstacles such as wood and dirt in order to reach mobs would suffice, though it would also make guarding structures using them...A potentially messy affair, to say the least.
I have a great idea for Diamond Golems, it has a negative and a positive side so as to balance it. Diamond Golems should shoot lightning, this would be extremely effective against most monster, but the negative is that they would super charge creepers.
no it most certainly would not be a great addition. ever since the iron golem was added a whole load of people decided there should be more. obsidian, diamond, gold, redstone and lapis golems were suggested and they suck. we dont need more golems.