I think rather than asking what fails you need to ask what fails and leaves you stranded.
Then ask what will be difficult or impossible to get at any garage or town.
So, for example, a serpentine belt leaves you stuck so carry a spare, but a headlight bulb won't leave you stranded and should be easily available.
Work through a list like that and see what you think.
Then look at, for example, the alternator. If checked and in good condition chances are it won't fail, but if it does you are stuck. Most likely failure is of the diode/brush pack so just carry that as a spare.
A head gasket can be replaced by locals/you if they have the bits, so carry a spare head gasket.
Just my 2p's worth, otherwise you'll need a lorry for all of the spares!