I mean, while the idea sounds cool...
1. I don't think it would work well with the game mechanics. The skill trees, what would happen to them if a sim is half vampire half human? I feel like there would need to be a lot of tweaks and programming changes that would take a lot of time to make, and even then we all know how TS4 is filled with bugs so I doubt it would work like it should.
2. It sounds extremely random and not in a good way. I know Sims is about imagination and possibilities, but having a half vampire half mermaid sounds like something a three year old would create a drawing of... it's too childish and, frankly, kind of bizarre.
I don't know, am I being to negative? There are too many cons, I think. I get that hybrids would create more diverse sims but... seriously? A vampire-mermaid? Mermaid-witch? I think we should keep them separate. Maybe in the next iteration of The Sims they could make it work if they decide on it from the beginning.
*Edit spelling error