Bukkit

Perhaps an integration with or like the way Infernal Mobs did would work, this could be just yet another verity of that spawn system.
http://dev.bukkit.org/bukkit-plugins/infernal-mobs/

I personally don't agree that this plugin should work (when it comes to spawning mobs) by occasionally replacing normal mobs with its own mobs. For that plugin, yes because that's part of its goal, but the goal of this plugin is to introduce new mobs.

It could be one way to spawn new mobs, but it shouldn't be the only way (otherwise it'd be a rather lazy approach). I myself prefer fine-tuned control, and would rather see extensive configuration options for how and when mobs via this plugin should spawn. Though, I wouldn't be expecting this anytime soon.