I have had good success with hardy pecan on bitter nut hickory. The bitternut is the fastest growing hickory and thus the only one that has a chance of keeping up with the pecan growth rate (which is much faster than shag or shellbarks and will cause uneven growth that will eventually be an issue). I am planning to graft medlar to hawthorn this year. I have had about 50 % success after 3 years with apples onto wild crabapples. It seems that very young ones do the best, with 5-10 year old trees doing less well in my particular case... not sure its meaningful information but just what I've noticed on the 20 ish crabapple trees I've grafted. Asian and European pears work fine on Bradford/Callary.
I will also try roses onto multiflora this year and goumi berries onto Autumn olive.
