whit a POD in the middle ages, what would it be like for the County of Flanders to remain independent. Would it countinue to be just a county, wich regions would it have, ... Also, in case it would manage to continue to remain independent for a long time, wich effect would it have on Napoleon's empire, the belgian revolution, the first and sewond world war.
"Remains" independent? It would necessit Flanders to have been independent in first place during Middle-Ages. Thy actually had a large autonomy from french kings, but never managed to get out of their suzerainty.
And that's the core of the problem, Flanders didn't had the capacity to be "independent" (as much this word is...anachronistic) as England, France or Norway were. But it could get "autonomous" for a longer time.
A carolingian Burgundy-like kingdom is unlikely, because while Provence wasn't really important, all the region between Seine and Rhine was for carolingians. It would be likely crushed easily.
So we'll have still a County of Flanders as base.
If we're talking of Flanders proper, and not any "state" situated in the same territory, a good POD is probably the "Bruges Matins" where Flemish elites managed to put out french garrison. But it would be hard to keep french kings out of there (as OTL showed) as they had both legitimacy and force to enforce their claim.
And eventually, Flanders would have likely fall into English suzerainty (didn't the Counts recognized Edward III as their suzerain in the beggining of HYW, after all?) for economic reasons : exportation of their tissues production and need of english wool.
Finally, Napoleon, Belgian revolution, etc. would have likely been butterflied by the POD.