Did RL Stevenson Plan ...

Many authors have thought so (and have published sequels to Treasure Island themselves). In my mind, the strongest case is the fact that only half the treasure was lifted from the island; that all the silver remained there. But overall I don't really think Stevenson planned a sequel, no. I think the open ending gives the reader the sense that "somewhere out there, there is pirate treasure buried" - even if he is fully aware it is entirely fictional. And likewise, Long John's escape is far better than the alternative: the reader doesn't really want to see him brought to justice, nor do we want to see him dead. I cannot think of a better ending than precisely what Stevenson gave us.