Question: Do we count Andrew as a Scooby or not? If not, he definitely should have been. Either way, I wish we saw him in more seasons as a scooby, though.
I would have loved to see a modified April (I was made to love you) become a part of the gang. I think that having her join the team instead of shutting down would have been an interesting bit, and she would have been very interesting. Think- a pretty, formal, china-doll like girl on the outside... but you tick her off and she can throw you through a window.
Also, I'm willing to bet she would come in handy on a non-fighting basis. She seems like as a perfect girlfriend, she would have to have a lot of domestic skills. She could be a babysitter for Dawn (might be comical, since April is probably naive enough to let Dawn talk her way out of the house), a good cook, and a housecleaner.