it's almost certain that the sequel will be based on Red Rackham's Treasure and The Calculus Affair. What else could it possibly be?
Err... well, any of the other books he appears in?
I'm with Elvind, I can't see that saying Calculus is in the film implies that they will use The Calculus Affair
for the next film; it doesn't exclude it, but it certainly doen't require it.
I've said before that I can see the logical progression of starting the next film with Rackham
, then following on from that, that the looted gold turns out to be the Inca horde, which contain the bracelet which Calculus puts on, allowing a segue into Seven Crystal Balls
(and a 3D CGI animated Rascar Capac!), and perhaps Prisoners
, or leave that for a further sequel (I could see that being an amalgamation of Prisoners
, with Rastapopoulos out to grab the treasure and ancient astronauts being woven into the Inca myth...).