At night, wild hostile wolves can spawn in packs (configurable sizes). These wolves behave a lot more like normal hostile mobs. Same despawn conditions, and they will only target players. When it becomes day, they will still despawn like hostile mobs, but they will become neutral to the player and can be tamed into normal wolves.
Night wolves are bigger, tougher, and see further than a regular wolf, and they bark in a lower, meaner tone. Every pack can also spawn with an alpha — an even larger, stronger wolf with a deeper voice that leads the group.
During a full moon, the wolf packs will spawn more frequently and you can occasionally hear them howling in the distance.
Wolves are territorial creatures, so keeping a tamed alpha around can help hold the wild ones at bay. Tame a former night wolf (or an alpha) and you'll earn an achievement for it.

Implement baby wolves.
Factor local area difficulty into spawning more groups of max size, maybe applying potion effects.
Possibly add a nether wolf variant for Halloween or as a rare spawn.

Better With Wolves