For the past dozen or so years, I have been performing various 'botanical re-mixing' experiments, investigating the capacity of native and naturalized plants to serve as 'scaffolding' for related species that offer higher food yields.
To date, I have successfully grafted numerous cultivars of domestic apple (Malus domestica) to the Pacific crabapple (Malus fusca), a wild species native to the Northwest Coast of North America. Malus fusca is a small tree that is very tolerant of poor drainage and salt spray. It often comes up in thickets of salmonberry (Rubus spectabilis), which is thorny and thus protects the young crabapple trees from deer browse. The crabapples, while edible, are relatively small and in contrast to many apples of cultivated origin, need to be dried or pickled to be enjoyed.
Malus fusca makes a useful rootstock, however, for those wishing to grow apples in wet, maritime climates. Malus fusca appears to show resistance to common diseases and the small size of the trees makes harvesting easy. I find its tendency to send up suckers useful as it provides a continuous supply of new places to graft onto. I have 'top worked' several older trees, using a technique called 'crown grafting' (which is a little brutal), and also by wedge grafting scions of many varieties to the ends of existing branches. These 'repurposed' trees now provide bushels of fruit on a site that used to be unsuitable for a conventional orchard because the soil stays waterlogged all winter.
I have also been experimenting with Sorbus aucuparia and Crataegus monogyna, which are now naturalized to the West Coast as well as the native Amelanchier alnifolia. All of these show some promise as rootstocks for Medlar (Mespilus) and European pear, with the Sorbus and the Crataegus showing the best graft compatibility. I haven't yet tested apples on these, but will do so in the future.