I think that not many series could benefit from an official sequel, though I wouldn't mind being proved wrong. I have my own ideas for Tamers and Savers continuations that may or may not (probably the latter) get turned into fan fiction. On the subject of Frontier, I agree with Tetra Seleno. Frontier was the only anime series where everything was actually wrapped up nice and neat. I mean, I do have another idea for a Frontier sequel fanfic, but that's only because I thought it would be funny to write a Digimon version of Stephen King's IT, and Frontier was the series that I felt would best lend itself to that. Xros Wars was almost too open-ended for me. I don't see where that could be taken.