'20,000 leagues under the sea" and its sequel

[Flambeau]

I wonder whether Jules Verne had planned beforehand to write a sequel, or if he wrote it simply because the book was so successful. I think it is not very likely he HAD planned to write "Mysterious Island" while writing the other. It so spoils all the mystery he leaves forever unsolved in the end of "20,000 leagues under the sea"!
I read somewhere that at first he intended Captain Nemo to be a Polish, who fought the Russians; but that his editor thought it more 'political' to change that. However, he does not tell us the Captain is an Indian who hates the English anywhere in "20,000 leagues"; we are told that only in the sequel. So that maybe he did plan to write a sequel after all... What do you think?
Well, be that as it may, I do not think "Mysterious Island" is a worthy successor to "20,000 leagues"; and I prefer to ignore it. How much more exciting to think that no one knows who the Captain was, what became of him and that he may be still cruising the seas, seeking his revenge! It must have been wonderful to read the book when it first was published, and sounded far more real.