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.
I'm up for a wider variety of villager professions, especially knights and archers as they could serve to protect villages that are too small to spawn an Iron Golem. Along with this, you could add a new type of structure that spawns with a village of certain sizes, in the form of hill forts for knights, and lookout towers for archers.
The archers would spawn in or around the towers, which themselves spawn on the boundaries of a village. The knight should remain in and close to the hill fort until night where they patrol a wider area. By hill fort, I simply mean a communal building that can be accessed by all villagers.
I'm not sure where I stand with waves of enemies attacking in increasing layers of difficulty, especially as this isn't really needed for survival, and can easily be built in creative with dispensers and a redstone timer.
I think, instead of a block, maybe this would work as a standalone game type, separate from survival, adventure, and creative. That way you could customise options for the game, such as initial preparation time, level of difficulty, number of waves, etc.
I'm almost certain I've seen this in mod form for the PC, but it would be such a significant introduction and change to the overall feel of the game that I doubt it'll ever happen. 4J have stated - a long time ago - that they hope to begin developing unique content for the consoles to compensate for the inevitable loss of content ported across that they can't support due to hardware restrictions. However, I can't see this being added.
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).