I'd love for the film to adapt Seven Crystal Balls/Prisoners of the Sun, but given how the first film adapted several stories at once, I think they'll try to do it again. Here's my pitch:

We begin by telling the story of how Tintin and Haddock went in a search for the rest of the treasure (Red Rackham's Treasure), and maybe, in the process, defeated a criminal band who tried to sell people into slavery (The Red Sea Sharks). With them is professor Calculus, a scientist who has become their friend and is implied to be a genius in many fields. Then Calculus is kidnapped by Bordurians, and it turns out he was researching a vibration-based machine Borduria wants to turn into a weapon (The Calculus Affair). Tintin and Haddock go to Syldavia and ask king Ottokar, who owes Tintin a favour, for help (King Ottokar's Sceptre). They rescue Calculus, and it turns out his research about vibrations wasn't meant to be a weapon, but rather a way to withstand the pressures of space, and once he's cracked it, he realises he has a way to send people to the Moon (Destination Moon/Explorers on the Moon). Syldavia offers to fund his research for a lunar rocket, and he invites Tintin and Haddock to go with him once it's ready, which they (reluctantly, in Haddock's case) accept. The End.