Ironically, both countries had relatively similar ethnic grievances,
Huh? After WWI, Germany became smaller. Hungary (not an independent country before WWI, but in comparison to its share of the Austro-Hungarian empire) became even smaller.
Romania? Larger. Much larger. Sure they still had some small minorities over the borders, especially in Yugoslavia.
But frankly I'd never call that a similarity with Germany plus Austria in the interwar period, which together were looking at parts of Poland, France, Czechoslovakia, Holland, Belgium, Denmark, Italy, Yugoslavia, Lithuania, i.e. nearly every neighbor save Switzerland and Luxembourg.