A new kind of villager that spawns randomly with normal villagers, or with a spawn egg, as different forms of defender's such as; Soldier, Archer, Dwarf, Wizard. Instead of hiding at night they remain at guard.
The Soldier runs in with his iron blade and armor. Or patrols the inside of the town.
The Archer runs to the flank with leather armor while shooting a bow. Or patrols the outskirts of town, and will retreat back to town to warn of danger.
The short Dwarfs run into battle with an iron axe and iron armor with high health regeneration. Or wanders around town randomly.
The wizard sets back providing health to allies and various potion's for defence, offence, and support. Or climbs and sit in towers.
Then have a program just like a monster spawn block that has various waves depending on the block used, for example;
The overworld spawn block sends its first wave on the second night. The first wave-
The underworld spawn sends its first wave on the third day. The first wave-
2 Magma Cubes
2 Zombie Pigmen
Then allow for the block to face a direction that the various monsters will spawn and march towards in waves of increasingly stronger monsters, while some of the monsters spawn and patrol a small area around the block.
This could allow for a kingdom war like system if the waves were spread out properly. I personally would find much satisfaction in something like this.
This sounds suspiciously like a certain custom game type created for the PC version...
Nevertheless, I don't really see this as being appropriate for vanilla MC. Iron Golems are meant to be the protectors of villages since the villagers themselves serve the roll of traders. Golems can be created by the player if a village is "too small" to spawn one naturally (though any player worth their salt will easily figure out how to expand a village's population).