I just read the below article, which gave me many ideas.
Gamespot.com | What Two Game Series Do You Want To Be In a Crossover Together? - GameSpot Q&A
I think a crossover between Bayonetta and Metroid would be absolutely amazing. Since both series have some degree of gore (mostly alien blood in the Metroid series), there wouldn't be much of an issue in that matter.
The snarky, comical atmosphere of the Bayonetta series combined with the dark, horror sci-fi atmosphere of the Metroid series would make for an excellent plot line. Furthermore, since both game series are action & shooting-oriented, I fancy it wouldn't be too hard to craft a unique and engaging gameplay concept that combines the spirits of both Bayonetta and Metroid.
Obviously, Bayonetta and Samus would be the two main characters. The supporting characters could be Jeanne, Rodin, Luka, and Enzo from the Bayonetta series, and Rundas, Ghor, and Gandrayda from the Metroid series. The baddies could be a mix of Space Pirates, Ing Warriors, Angels, and Infernals. Imagine Bayonetta fighting Ridley or Kraid--or Samus fighting the Golem or Sapienta.
What say you lot?