Ok, so I have some suggestions to throw out. This might be a bit long, so bare with me.
Current System Improvements
Vampires- Ok, I believe a few things should be touched up on this a bit. I'm not going to touch on the sunlight thing, because the community beat the idea horse last time it was brought up. For one, there should be a way to control your vampire children more. For example, having them follow you, commanding them to feed on something, or even leveling them up so that they can reach your state of power eventually. Vampires should also have a sort of mist form, which would make the player untargetable for a short period of time. Fire would still hurt though.
Lycanthropy- There should be unique appearances for werewolves. Example, different coats of fur, eye colors, etc. It's a weird request, but I feel it'd add to immersion. Werewolves should be able to inherit some of their strength and speed in human form, and maybe have a sort of scent mechanism.
Demons- Dealing with demons feels a bit lackluster, and there's really no drawback to it. All you do is summon one, trap it (hardest part really), then use it for trading and banish it when convenient. They need something more. There needs to be something at risk here. You're dealing with a nasty creature here. Demons are monsters, tricksters, liars, but the mod does very little to show it with the exception of the monstrous aspect. Along with that, their appearance is rather lanky, easily comparable to an Enderman just because of the model alone. Basically, demons need a revamp.
Mandrake Root- It needs more uses. I was thinking maybe if you Shift + Right Click a bed it hides mandrake root under the bed (ripping off Merlin a bit) causing nightmares. I tried using demon hearts once, but the noise makes it obvious. The root could be discovered the same way it is placed, but does anyone really shift click their bed?
Death- He needs to show up more. I think it'd be pretty interesting to see a spectral embodiment of death appear nearby someone when they die. Also, Death is really underwhelming. I mean c'mon, he's Death. Literally, the embodiment of, well death. But, he can be simply killed, mind you killed, by chucking instant damage potions at him.
Dragons- Yes, I know Draco Animus exists to fill this gap but it's, just...no. I am thinking grand scale monsters, lying on a bed of gold. You make them mad, they come out of their randomly generated cave, and devastate the nearby area. Villages, forests, even mountains, nothing is safe. They don't just hunt you though, oh no, you woke him or her up kid. Now it's going to wreak havoc on the world. Kill it, get a nice reward. Dragon blood, tooth, maybe bones or something. Maybe if you play your cards right, an egg. Then, become a dragon rider. But hey that's merely a wild fantasy of mine.
Marionette Ritual- Fairly simple, perform an infusion on a puppet and then you can control the person with the puppet for a short time. They can however resist. However, it works wonders with mobs. You can even see from their perspective.
I'll suggest more when I have time. Ta ta for now.