The Franco-Belgian Accord, which lasted from 1920 until October 1936 when Belgium declared itself neutral, included an article which 'discussed greater integration of frontier defenses' (according to Wikipedia).
The Maginot line was started in 1929 and covered only the France-Germany border. The Ardennes forest was considered impenetrable and was thus not protected.
The reasons why the French didn't extend the line (initially at least) along the France-Belgium border are clear enough (they didn't want to offend their allies the Belgians). The belief was that the French army could defend the shorter (non-Maginot) area, but this seems to be contradicted by the extension of the line after the Franco-Belgian accord fell apart (though the extended section was not of the same strength as the original section).
Prior to Belgium's withdrawal from the accord, was there ever a plan to extend the line along the Belgium-Netherlands border as part of the aforementioned discussion of 'greater integration of frontier defenses'? If not, was this because of the cost? Or was it because the Belgians (especially the Flemish population) didn't want too much French influence? Or was it because the Belgians didn't want to offend the Dutch? Or perhaps a combination of all three?