All of the ideas I'd been considering have run into Euro copyright problems, lack of useful sources, etc. For example there's a book I've heard of that I'd REALLY like to adapt as a game but only one copy is known to exist, it last changed hands for £15,000, and I doubt very much that the owner would let me scan it. Plus if I ever get round to reading it, it might turn out to be crap....
More seriously, a lot of the more distinguished authors of the late Victorian and Edwardian era turn out to have died after 1934, which means that they are still in copyright in Britain and Europe. George Griffith and Conan Doyle are exceptions, but I've already written the Professor Challenger RPG, Sherlock Holmes simply won't work as an RPG, and I've already used the best Griffith stuff - his fantasy isn't nearly as good and none of his other books seem to be available.
So I'm thinking of various possibilities, most notably something based on William Hope Hodgson - I've already done Carnacki the Ghost Finder in FF IV, but there's also The Night Land, The House on the Borderland, The Ghost Pirates, and various short stories. The problem is that I'm not a great fan of The Night Land, which looks to be the most suitable as an RPG setting; I may find I like it more if I read it again, but it doesn't seem likely. Ghost Pirates could tie in with the time travel part of FF IX quite well (so could House on the Borderland), but the model of time in these stories doesn't quite fit with what I said in FF IX. Plus I'm not sure I want to write more time travel again right now.
Kipling's still a year or so in copyright - I published FF I when he was out of copyright before the law changed, and appear to be legally OK provided I don't change anything - so that rules out The Jungle Book or any of his military or engineering-driven fiction. I'd really prefer to do something with a stronger scientific romance / SF feel next time anyway. Wells is in copyright until 2016, and most of the good Verne translations are still in copyright by the translators.
So... any suggestions?