Wow I was thinking of making a vampire mod that has much of what you have done, so maybe I'll just spew out my ideas and hope you can consider implementing them.
NPC's waking up when feeding on them, (mentioned above) or possibly dying, see below.
I don't know if your mod has various vampire starvation stages, etc, but most oblivion vamp mods do. It would be cool if you had to feed once to reduce your craving by 1. So if you are at level 4 starvation, you would have to feed 4 times to get back to fully fed. Now if you did this on ONE target, they would definitely die after 2-3 feedings, or wake up screaming, etc. This could alert guards and make feeding more of an art of stealth or seduction.
Also I think each time you feed on a person they should get a token placed or whatever so that if you feed on them 3 nights in a row they might die from blood loss, and then perhaps you have a small window of time to revive them as a vampire, who would then follow you like a companion.
The player would no longer be able to just keep feeding on the same bum or someone in the mages guild, etc without raising suspicions or killing people, etc. The vampire would have to be on the move and stealthy, at least in the beginning until they can acquire cattle to feed on, etc.
I think if sun damage is great enough, it should make the player on fire and make the vampire hissing sound.
I like extra buffs during evening hours.
I'm not a fan of daywalking mods, I think a vampire should be confined to darkness, but as a higher level vampire, I believe dracula could walk in the daylight, but he was much weaker. Perhaps make it so you are weaker than a normal human with drains on all your attributes during day hours or if you are outside.
If you need any help, I have 13 years professional game development experience, (and modded before that) and can model, texture, animate, create sounds, voice acting, quests, etc. PM me if interested, I've been wanting to get back into some kind of modding. I can script a bit, but nothing like you can, I'd rather focus on content creation
Just ideas, I have lots more, but I'll save them if you are interested
Your mod sounds great I'll be trying it out!