With a POD before 1830, have Belgium stretch from its current northern borders to the Seine.
Clovis of Merovais loses his battles and establishes a Frankish state north of the Seine instead of projecting power through to central France. Some other petty kingdom becomes ATL France (burgundy? occitania?), while the OTL France is displaced into north France/Belgium.
voila, a french state between seine and holland.
OR
when Charlemagne divides his realm, neustrasia (france) is given the entire netherlands coast and lotharingia is shifted down and west into burgundy to compensate. Basically reshuffling land ownership. At some point the northern half of france is separated from the rest of france.
Or the same but with Lotharingia having a bloc of land in the low countries and northern france, while france instead gets the southern half of france and italy.