The villain
I was considering something: In all the films, we've never really seen Indy face off against a true equal, a true rival, someone almost totally like himself. An equal in ability, strength, intellect, etc.
Belloq was similar to Indy in terms of having a gray sense of morality and a love of archaeology, but he got his hands dirty. He'd have other people do his dirty work for him and reap the rewards of others' hard work--The type of person who'd hire someone like Indy to do the dirty work for him. He wasn't a action type of archaeologist like Indy in that sense, like a man who would actually risk life and limb and go into a tomb or cave full of booby taps to get what he wants; he'd have some cheap labor do it for him. He regarded archaeology as something to be dealt with precisely. His little remark about how the Nazis would "use a bulldozer to find a China cup" is a great insight into how he views archaeology and how it should be treated.
Indy's method for better or for worse is like going in with a bulldozer to find a China cup. And Indy in this instance is the bulldozer. He's desecrated and allowed for the destruction of ancient tombs just to obtain golden idols, for example. While he might hire help to AID him, notice that none of his hired help ever does the work for him--unlike Belloq. Belloq is a user; Indy is usually the person being used. Think in Raiders--Belloq is "working with"--using--the Nazi government for his own ends, and really using those around him to do his dirty work; he acts as the director of the Egypt expedition. Indy on the other hand is WORKING FOR the US government for their ends. He's not using them, they're using him.
Donovan wasn't even really an archaeologist, at least, that element of him was never really emphasized. He was a scholar sure, but he seems much like a Belloq, albeit less sophisticated and even more ruthless and evil. I can't see Belloq shooting Indy's father to get Indy to act; Belloq, while a morally gray character, never struck me as evil, unlike Donovon. Belloq respected Indy to a certain extent, whereas Donovon sees Indy as just another pawn, just another tool to help him reach the end. Belloq is probably the only villain in the series, along with Spalko who isn't truly traditionally "evil."
Donovon mentions engineers working in Ankara unearthed the Grail Tablet. I'd bet those "engineers" were men like Indy, men willing to go into the mountains north of Ankara "while excavating for copper" and get their hands dirty while Donovon sat back, relaxed and got his goals accomplished. Donovon was just a snake who was manipulating everybody from Hitler to Indy. Mola Ram was a psycho cult leader and Spalko was a Soviet agent.
The only one who comes close is Elsa in the sense of being a field archaeologist, and even then, she's not a straight villain or a major villain; She goes from being the love interest to the villain to the very end, never really sure of her feelings, only really on her side. She seemed to like Indy, but then again, who knows? She slept with both Indy and his father to try to get what she wanted and used the power of those around her to further her own ends.
It'd be interesting to have Indy's final villain--major villain, or one of the major villains--be a character almost exactly like himself in many ways. Perhaps a longtime rival--A "Forrestal" type. A guy who like Indy doesn't sit back and let others do the dirty work but goes in and does it for himself. A fighter and gunfighter just like Indy. Maybe younger than Indy, perhaps. Or, maybe an older version of a Fedora type character--a longtime rival of Indy maybe. A truly shadowy reflection of Indiana--moreso than Indy.
Indy's faced off Nazis; He's face off against Soviets; He's faced off against Thuggee Cultists--But he's never faced off someone truly like himself. That'd be the ultimate villain in a final film--a true equal. The Moriarty to Indiana's Sherlock. This route would be especially interesting to go down if they do decide to kill Indy on film, but even if they don't, it'd still be an interesting angle and would help take the film back to the grittier roots of Raiders.