Hmm... arguing realism here...
(Raiders) Map Room
A small set, but one that gives Sallah and Indy as well as some cute pieces for the Staff of Ra. On top of that, it's an iconic scene in the movie.
(Raiders) German Soldier Set
A 'troop builder' for Indiana Jones, consisting of a handful of German soldiers with an officer and perhaps a small vehicle, like a one-map 'Jeep' similar to the Jungle Tracker.
(Raiders) Bantu Wind
Seriously unlikely, but it could easily be a redeco of the existing 'cargo boat' set from Lego City. Marion, Indy, Katanga, crewman.
(Temple of Doom) Rope Bridge
It's really unlikely that we'll see a lot more of Temple of Doom sets, considering. But the Rope Bridge would be a safe bet since you can pack nearly all the minifigs you need in it, and reuse some earlier 'jungle themed' parts. Mola Ram would be the highlight, naturally.
(Temple of Doom) Lao Che's Transport
Pretty much the only one that would warrant a big set now that the Temple of Doom mine-cart chase is coming out. This could easily be a rebuild of the existing passenger planes.
(Temple of Doom) British Riflemen
Similar to the other troop builders, but this time with British troops and an elephant. Throw in Captain Blumbard too.
(Temple of Doom) Elephant Ride
Three elephants, some bats, our heroes, but really a recycled set from the Adventures line.
(Last Crusade) Desert Tank
This is so highly demanded that it's impossible to imagine that it's not going to come out eventually. It'll probably be the 'big' piece of any future line.
(Last Crusade) Saint's Tomb
A small set, consisting of Indy, Elsa, and a small 'room' for the tomb and a bunch of rats.
(Last Crusade) Zeppelin
To be remotely feasible, you would build the room, the frame, and then drape a plastic 'cloth' around the body. Really not sure how well this work work out, honestly.
(Crystal Skull) Russian Troop Set
Like the German one above, only in gray.
(Crystal Skull) Motorcycle Chase II
Mutt on a black bike with Professor Indy, with a 1950's era roadster chasing them with Soviet 'suited thugs' inside.
(Fate of Atlantis) Knossos Shrine
Small room (some action feature) featuring Indy with Sophia, along with some 'German Soldiers'. Include some gold 'orichalum' mini-bricks.