Although it was before my frequent flying days, I remember that in the mid-90s DL tried to establish a hub in FRA. Or did I get that wrong.
AA had kind of a hub in GRU, with connections from JFK, MIA and DFW to MVD, GIG (?) and ASU. Now they don't fly ASU anymore and serve MVD via EZE.
What would qualify for a hub operation anyway?