Hey lore-gang

I'm actually looking for neat ideas here, as opposed to factual lore, as I don't believe its so much as mentioned. Serana, without mods, will never transform into her Vampire(ss) Lord(Lady) form. This... irritated me, so i've fixed that. However, it occurs to me that if I ever want to play my vampire characters in any future TES games (and vampires fit that like few others) the option for my characters to transform will most likely not be there.

So, i'm looking for roleplaying reasons why a vampire would not use the (supposedly) powerful form. That gives me something to attribute to Serana not transforming while an Elder Dragon is chewing on my head, and for my own character in the future.

My first thought is to go the White Wolf direction, in that each transformation into a vampire lord costs a little more of the vampires remaining humanity. It just seems a little basic, though I suppose it will work if nothing better appears here. I could take it a little further, and roleplay it that the vampire loses fragments of soul/themselves that go directly to Molag Bal. That might work.

Any thoughts? The more unique the better, really.