I propose a stage for vampires called "Blood Gorged" which is a stage that kicks in if you feed when you are stage 1 (already fed). In this new stage, you still look liek a stage 1 vampire (maybe a bit less pale, but still paler than being alive) with the same freaky eyes and all - but this stage lasts 3 hours and you take only 20% extra damage from fire. -HOWEVER- to keep it balanced and more for personal preference and RP/appearance - all vampire abilities' costs are 200%. So to break it down:
Blood Gorged:
Only 20% extra fire damage
Slightly more "alive" appearance (note slightly)
All vampire abilities cost 200%
Stage lasts 180 minutes
As for the graphical tweaks, I figure all stages need fangs and stage 4 vampires should get claws like the khajiit have on their hands and feet.
Minor things but adds a bit more to vampires. ZOS already mentioned that they were looking at some alchemy stuff for RPers (I am assuming blood potions that work like bottled feeding). Maybe even add a option for a player to willingly be fed off?