If I may, I have some faction suggestions and one or two things you probably forgot to say:
Your faction (obviously) wouldn't attack you (except bosses). They can't kill your NPCs, either, since you can move them there (ok, maybe evil biomes vs Dryad, who will have a more offensive protection method).
Your faction's enemies (Corruption and Hallow vs Crimson in your example) can also invade you, and will travel to your spawn point like an invasion. Only one biome can do this at a time.
For a faction to win a faction battle, 150 of their opposing biome's creatures must be killed. The play may interfere here. As well as this, if 100 of each are killed with the player having killed at least 50% of each side, neither will win. If it's within 5 points when one side reaches 150 (like 150 points to Crimson, 145 to Hallow) they will continue for a minute more. If there is 6 or more points in-between them after this minute, the side with more points wins. If not, neither side wins.
If you have both evil biomes in your world, meaning two biomes may oppose your faction, they will focus on killing you and then killing each other, since they would hate you even more.
Building homes in your factions territory and leaving them empty will not cause NPCs to naturally move in. As well as this, you could get more respect among your faction, since you're giving homes to the many creatures of your biome. Humanoid creatures based off of your biome will move in to these houses, and will be like NPCs. Also, like the faction leader, they will give you quests, which are mostly revolved around spreading your faction or killing its enemies, but may also be about giving them items involved with your faction, such as the Bladetongue.
While I agree with killing your faction's boss giving you points, this shouldn't work with the creatures. You can still kill them in case you need souls and can't be bothered to spread the other evil in your world, but this will turn all of the biome's creatures currently spawned against you. If you kill enough of just one type of your faction's creatures (e.g. your faction is Crimson and you kill 100 Face Monsters), that creature will turn against you. If this is done to every creature of that biome, you are no longer part of that faction, and, if wanting to join again, will have to do quests before you can receive faction bonuses.
These are just suggestions though.