I don't now about giants, but you could certainly put in a good word for such Marxists as
Karl Korsch, Paul Mattick, Franz Neumann, Roman Rosdolsky, Evgenny Pashukanis, Anton Pannekoek, Jindrich Zeleny, Istvan Meszaros, Evald Ilyenkov, Lewis Corey, William Blake, Abram Leon, Jakubowski, Preobrazhensky, and of course Trotsky
But I'm interested to know who are the great Marxists who are non- Europeans. I know of a lot of non-European revolutionaries who I would call great men and women, like Carrol's list "Cabral, Mao, James, Ho" but not many who developed Marxism. If we asked ourselves what was Mao's contribution to Marxism? Wouldn't you have to say that he muddied the waters about the potential of peasants to build socialism? His contribution to China's liberty I happily honour, but his Marxism was a rhetorical cloak to his (proper) nationalism.
-- Jim heartfield