As an author of a vampire series where the main character is a human woman, I will always be faced with the question 'will Serenity become a vampire'?
In many ways, it's the only real way a human and a vampire can ever be together forever. However, I still find it somewhat disappointing when the main female in a vampire series is turned. For example, I never saw Elena in 'The Vampire Diaries' becoming a vampire and when I realised that was what was going to happen, I couldn't help feeling let down. As soon as the human becomes a vampire, for me, the whole series is over. The point of a vampire/human relationship is all the contrasts: fragility vs strength, warm vs cold, mortal vs immortal. As soon as that is taken away, the difficulty and angst within the relationship vanishes, and often so does the storyline.
Another example of this happening was in the 'Twilight' books. Bella ends up in the series as a vampire, yet so much in the previous books were about how Edward was trying to save her soul and how he'd rather leave than turn her.
For me, I would like to think a human can hold their own, even against vampires. Yes, there are issues with the 'never growing old' thing, but perhaps there are simply more clever ways to combat that problem. I can't promise how my 'Serenity' series will end - I don't even really know myself (I have some idea, but my ideas have a tendency to change as the book is being written). Some of my readers have told me they hope Serenity will become a vampire, but I'm not sure I want that to be how the series goes.
Serenity is human and that fragility is an integral part of her character. To change it would feel like not only a lie, but an easy way out.