That's a lot of questions - I hope I can answer them...
1) There's no deadline for sign-ups and we'd love you to sign up!
2) As long as the original author(s) is okay with sequels, and as long as your fic is 20K previously unpublished words, complete and standalone (in the sense of working as a story in its own right, without a reader needing to have read previous fics or needing to wait for future parts for it to feel complete) then it would fit the rules for the Big Bang. If you then want to add extra bits/sequels/alt endings afterwards, that's up to you! We only require one ending by the deadline, although extra ones sound intriguing :)
no subject
1) There's no deadline for sign-ups and we'd love you to sign up!
2) As long as the original author(s) is okay with sequels, and as long as your fic is 20K previously unpublished words, complete and standalone (in the sense of working as a story in its own right, without a reader needing to have read previous fics or needing to wait for future parts for it to feel complete) then it would fit the rules for the Big Bang. If you then want to add extra bits/sequels/alt endings afterwards, that's up to you! We only require one ending by the deadline, although extra ones sound intriguing :)