The large majority of mechanical failures I have seen myself or others have on tour are wheel related and mostly spokes with hub or rim issues were much less frequent. I generally carry a chain tool, spoke wrench, and a Unior cassette cracker to remove a cassette if necessary. I also carry a few spare spokes and the usual stuff to fix flats.
Most stuff on a bike can be fixed in a auto shop or home/farm shop in a pinch. Sometimes it may be a less than perfect fix until you get to a place with the proper tools.
Anywhere I have toured on road I would always have been able to hitch a ride to a bike shop if I had to. Sometimes it might have been over 100 miles, but in those places folks tend to be especially willing to help and offer rides or other assistance. Worst case getting to a decent sized town would allow me have a frame or even another bike shipped from home even if that was my around town beater or my MTB. In the latter case I could put the broken bike in the box and send it home.