During the days of the Hansa, Low Saxon was used as a trade language all the way to the Baltics, and one may argue that the northern coastal plain of Germany and Poland should be defined as 'low country'. It's just as flat as the Netherlands. Considering the vulnerability of a surviving Lotharingia, this kind of thing might be the better way to go: a contry that unites all Low Franconians, Frisians and Saxons (including those speakers of Low Saxon who overwhelmingly settled Pommerania and Prussia during the Ostsiedlung). We'd be talking about the OTL Low Countries, plus roughly everything blue on
this map.
Such a country could be economically powerful. Quite possibly powerful enough to enforce its culture on the parts of the Low Countries that remained French(-speaking) in OTL. I think this country could realistically extend from the Somme to the Memel. It would have Calais, Antwerp, Ghent, Brussels, Liege, Amsterdam, Rotterdam, Cologne, Münster, Bremen, Hamburg, Hannover, Berlin, Danzig and Königsberg-- and a score of other cities that are economically significant on an international scale. This would be the economic powerhouse of Europe. Potentially moreso than OTL Germany.