Well, as I understand it, Demitri was the main character of the first game, Donovan was the star of the second and Morrigan was the main character of the third one.
Personally though, I don't consider a lack of Donovan to be a weakness. I just don't like the character. The whole brooding holier-than-thou Dhampir monster killer thing just doesn't do anything for me. The strength of the franchise always seemed to me to be the idea of monsters fighting monsters. Better to keep the monster hunters out, in my opinion.
One thing I found curious was how they managed to adapt the concept of a succubus for kids' TV. From what I've seen, their take was for Morrigan to be sort of a demonic version of Rogue or Sauron from X-Men. She hypnotizes men and then drains their life energy by kissing them.