I'll try and answer
I can't make any changes yet (no PC for Modding), so if I forget, please remind me again
1. Would it be possible to add the vampire feeding animation when feeding on a person/corpse?
Yes/No - I can add it in for feeding on people, but was getting some odd detection issues, so removed it temporarily to try and work out why.
2. I think it would be more balancing to remove the option of "replenish" when feeding on an NPC - this makes it too easy to replenish all your blood (Which at high levels is significant (200000+)), so instead you could sip (1000) or replenish all, but kill them in the process... that would add more of a reason to ever use the drain option on an NPC that you rare not trying to assasinate, bringing out more of the "monster" in you.
I'll take it under consideration - I see your point, but would like to know what other people think first.
3. I never see my vampire face - is there a reason why you don't use a 'mask darkness' power to switch between faces (like a bug etc?). I find it more immersive as a vampire to have a vampire-face, but that's just personal.
Technically a bug, but with Oblivion: Vampire faces can cause issues with save files/character gender, so I avoid it as much as possible (it is only added when you feed, for a few seconds); some people also don't like it.
4. May be a good idea to switch vampire stat bonuses from 'disease' to 'ability', otherwise they get 'cured'. Also I still get diseases even though I'm 100% resistant to disease - does OOO/FCOM or other such mod overhaul diseases to make this the case?
No can do, potentially: Attributes "Wrap around" passed 255-- so if you're dealing with something high-end (depending on your stats) your 500+strength Vampire suddenly becomes a 10-strength weakling
Diseases *should* fix themselves, though, via the scripting-- if you're not getting it, let me know. As to contracting diseases... Mayhappen it's to do with a disease stopping diseases, could be an Oblivion bug... I'll need to check it out.
5. If you drain a corpse, could it be possible to make their skin crinkly, discoloured etc, so they look rather dry and lost for blood?
Should be do-able
6. Have blood drain whilst you wait/sleep - I think this would greatly add to immersion and increase your reliance on blood.
This was a design choice, personally: with Vanilla Vampirism, you have to monitor your wait/sleep time, because of the 24-hours. I felt it would be more enjoyable to ignore sleep/wait/fast travel, so that the Player does not have to focus on this too much. If there's demand, it can be added, I believe.
7. Some awesome transformationary superpower at the highest level - like Markus from Underworld Evolution - not necessary, but would be wicked!
Maybe. In an earlier version, we had Dracula-style wolf-transformation, but people got CTDs with it for some reason (I got them originally, but seemed to fix it-- why some people were still getting it, I was not able to work out, and when I rebuilt, I removed the feature sicne I assumed it was something to do with faulty-meshes, since the code "should" have worked perfectly). I tried adding Wings to flight mode, but got the same issue, which is confounding