The main difference would be that the Vikings are a bigger threat in 867 than in 936. You'd know better than I would how troublesome Vikings are in 936. In 867 expect many different Norse rulers to be sailing around raiding you and launching county conquest wars any time West Francia looks weak, which it often does. The down side of rich lands is that people will want to steal your riches if given half a chance.
In 867 Boudewijn has Brugge, Yperen, and Gent, and two de jure provinces are not his vassals at all. In 936 Arnulf has Brugge, Yperen, Gent, and Artois, and has vassals in Guines and Boulougne.
Boudewijn already has three sons, Arnulf just has a daughter. Both are married to Karlings (van Vlanderens married to nearby monarchs was rather common), Boudewijn to the daughter of the King of West Francia, Arnulf to some sort of cousin it looks like.
Arnulf has Otto's Germany to think about in the future; Boudewijn has the 867 fractured landscape with a stronger Lotharingia, East Francia in a weaker position than Otto's Germany, etc.
Overall I'd say 936 is probably be safer bet. Though 867 would be more exciting, if you're into that kind of thing. I didn't look at 1066 but it was a fun game in the original CK (that was my final CK game, taking Flanders independent, recreating Lotharingia, and picking up the Kingdom of Burgundy as well). In this game a 1066 start means having to deal with the HRE on your border, since it won't conveniently implode like in CK. But maybe with the player helping out as Flanders, AI France wouldn't suck so much and could actually keep them in check? I don't know...
Another idea for a Flanders game in 867 would be to start somewhere in Scandinavia and conquer Flanders.
In 867 Boudewijn has Brugge, Yperen, and Gent, and two de jure provinces are not his vassals at all. In 936 Arnulf has Brugge, Yperen, Gent, and Artois, and has vassals in Guines and Boulougne.
Boudewijn already has three sons, Arnulf just has a daughter. Both are married to Karlings (van Vlanderens married to nearby monarchs was rather common), Boudewijn to the daughter of the King of West Francia, Arnulf to some sort of cousin it looks like.
Arnulf has Otto's Germany to think about in the future; Boudewijn has the 867 fractured landscape with a stronger Lotharingia, East Francia in a weaker position than Otto's Germany, etc.
Overall I'd say 936 is probably be safer bet. Though 867 would be more exciting, if you're into that kind of thing. I didn't look at 1066 but it was a fun game in the original CK (that was my final CK game, taking Flanders independent, recreating Lotharingia, and picking up the Kingdom of Burgundy as well). In this game a 1066 start means having to deal with the HRE on your border, since it won't conveniently implode like in CK. But maybe with the player helping out as Flanders, AI France wouldn't suck so much and could actually keep them in check? I don't know...
Another idea for a Flanders game in 867 would be to start somewhere in Scandinavia and conquer Flanders.