Iron Golems should have a level up system.

 https://www.reddit.com/r/minecraftsuggestions/comments/cnckj5/when_iron_golems_kill_monsters_they_should_have_a/

Top prevent the iron golems from being op, the level cap would lower based on how many iron golems are in a 4x4 cunck radius.

#Experience System

The way it would work is that different mobs have different experience values for iron golems. For example, a zombie would give 10 experience to the Iron Golem. When it reaches the required amount of exp, it levels up.

#Levels

All iron golems (player built) would start at level 1 while village golems would start at level 2 or 3. Every time an iron golem levels up, it would have increased health and attack strength. Every 3 levels would give it a random piece of armor specifically made for iron golems and the maximum level would be level 15.

#Armor

This would only be armor of steel or chain mail (to prevent it from being overpowered) and cannot be crafted. The armor would have some of the moss decay like an iron golem and also obviously protects the iron golem even more and as said before,would only be granted every 3 levels.

I’ll add anything else that I find interesting.

#Extra

When an iron golem levels up, there is a little animation that plays and the golem gets a bit bigger.

When the iron golem dies, it drops the special armor which can be equipped to another iron golem. However, the iron golem on the receiving end would have to be level 3 for 1 price of armor, 6 for another all the way to level 15.