some 'shaman' in disguise?
Just thinking about it... how about making Indy's next villain a dangerous shaman who has a distinctive power to 'force' out bones and the inner body through skin (killing people by "pressurizing" their blood and muscles until some of their veins burst and they collapse - okay VERY graphical and gore-y indeed but so are melting nazis?), and that via a supernatural McGuffin like old Mola with the sankara stones.
I'd absolutely love this villain then to be playing tricks on Jones, planting false leads with dangerous turnarounds. It would of course be somewhat corny but what if the villain was also a master in disguise and uses a believable mask (of actual human flesh sticking on via some kind of 'supernatural' adhering substance, which glows when tampered with?) and voice deformer - appearing to Indy as a faithful and unsuspecting helping hand who only unveils his true identity at the perfect time of revenge and treasure-grab
.
Why not... some Korean person (so there could be a Cold War runner-up to the Korean War in there aha!) and they land in Cambodia, hussling with the deadly Khmer Rouge.
At any rate,
Javier Bardem to play this mischievous, chilling dude with a truly fake (corpse's) face.
I mean, didn't you guys get spine chills with that scene in
Skyfall where he removes that jawbone prothesis