From the preceding comments, it is obvious that there are a lot of grey areas in this plagiarism/inspiration debate, and that we are never going to come to a complete consensus on this issue. As I've said before within this debate, we are all inspired by the stories we read, and the films we watch etc. If someone wants to write a sequel to something they've seen on the Plaza, then so long as its a different story and not the same plot rehashed, then I don't think anyone can complain too much. It might be courteous to inform the original author, but not everyone will.
Recently on the BBC in the UK, there has been a series called 'Dickensian' which (I'm led to believe) is a sequence of stories using characters from Charles Dickens' novels but with new 'what happened next' storylines. Or in other words, a sequel. Whilst the purists might look down their noses at this, I haven't actually heard anyone shouting "plagiarism!!"
I think we need to concentrate on eliminating the real plagiarism, in which entire stories are taken by others and claimed as their own work, whether that be word for word, or merely plot theft.
I'm sure some will agree with me on this, whilst others will have differing opinions. But as I said at the start, I don't think we'll ever all totally agree where the line gets drawn between what's acceptable and what's not.