With a huge caveat that I've not exactly cycled in remote places, and never had anything bad break on a touring bike (except a QR seatpost clamp)..... A derailleur would be nice, but let's face it, you can operate a bike without a front or rear derailleur (tough I know depending on terrain). Forgetting the obvious of spare tyre, inner tubes, multi-tool, lube, spare spokes and spoke wrench, cables, puncture repair kit, etc, etc my vote would go for:

-portable cassette remover (now how are you going to replace spokes on the driveside -which is usually the most stressed) anyway?
-spare QR front and rear, and a seat clamp bolt/QR. One of these babies breaks, you're in big trouble without a spare.
-cable and housing cutter
-housing and cable ends (can you just see your cable fraying away in the desert?)
-spare brake pads
-spare pedals
-spare friction shifter (but just one if its for a flat handlebar as you can use it either side, and just the right side downtube shifter -I reckon in an emergency you'd make do with just your middle chain ring).
-length of chain
-spare tyre (just because it sounds like you won't be able to get one easily).

I might also add one spare brake lever, cassette/freewheel but I think that's pushing it for me. I admit alot of this is based on the practicality of carrying this stuff and the issues faced if one of the items on your bike did break, rather than probability of failure. None of the above should be that unwieldy or heavy -I suppose if it made you feel alot better, then add in a front and rear derailleur (though of course, no matter what you take, it's a complete waste of time if you don't have the means to take the old one off the bike and replace it with the new one!!!) Putting aside practicalities, I'd add a saddle and seatpost, and handlebar and stem, but that is just getting plain silly to carry. After lugging a spare tyre around half of Europe once, and seeing all the bike stores there, I decided then and there that unless I really was going to the middle of nowhere or where bike stores are a real rarity, the spare tyre can stay at home! I really believe if you plan for *everything*, you'll be wasting your time and just be overloaded.

FWIW items that I've broken:
helmet (ouch!)
rear derailleur (once, mtbing)
handlebar (bent, but not broken -didn't exactly instill confidence)
various spokes
badly cut several tyres, with just one that couldn't get me home.
brake lever (mtbing)
rim (overloaded it, it wasn't meant for a heavy load!)
dinged and bent frame
cables
clipless pedals (in the middle of winter on a very very cold day, I clipped in.... and crack!).
rear freewheel (pressed down on pedal -just exploded!)

I'm sure there's more, just can't remember. Generally, I've had extremely good luck with bike mechanics.
The one thing I've always feared mightily was somehow breaking a seatpost or seatpost saddle rail clamp, and then being impaled on it (brrrrrrrr gives me the willies thinking about it) but it's never -thank goodness -happened. Perhaps alot of this can be put down to buying mid-level componentry that's not necessarily lightweight but at least strong.