Would Dutch and Flemish people be offended if era II civ was simply Flanders and Netherlands was its era III successor? I mean I am aware of the former later forming Belgium, not Netherlands but it has been culturally very close (I'm not even sure if culturally very distinguishable from Holland at the time of the middle ages, feel free to correct me) and Flanders was an economic superpower of the middle ages and early 16th century, being somewhat fluently succeeded by the Dutch from the late 16th onward. It even has some historical sense, with Dutch golden age being partially fueled by many Flemish and their capital migrating to the republic from Spanish Netherlands.
Technically we were once one people and in a way still are, but a lot of us converted during the reformation (north and south) to protestantism and then we came under Spanish catholic rule who didn't like that and wanted to correct that.
So a revolt and (a 80 year) war broke out. During the war, Antwerp (then a reformed powerhouse) was sacked by the Spanish and a lot of the reformed people in the south migrated indeed north and most of them to Amsterdam which blossemed. The people in the south that stayed under Spanish rule were eventually converted back to catholicism.
And even though there were a lot of catholics still up North, under the protestant rule north and the catholic rule South our cultures shifted in different direction a bit.
We still use the same standard language even though with slight different accents and some different words, but we all now the meaning of those. We can hear most of the time immediately from which side of the border we come from.
Say it's a bit like British and American English and as long as neither of us speaks in heavy dialect (which can differ a lot) we understand eachother with no problems.
Mostly in English media you see a lot of use of Flemish when they talk about language in Belgium which is nonsense (the official language in Belgium/Flanders is Dutch)
We joke about eachother (dumb and greedy) sometimes but for the most part you can consider us very good friends.
I don't see us uniting anymore though, even though most protestants in the Netherlands became irreligious and catholicism is now even in the Netherlands the biggest religion, the Flemish people have been on their own for centuries (technically with the Waloons though, which had a lot of influence on the culture, like Brussels turning from a Dutch to a French speaking city) and even if Belgium were to split up I think we would just get an independent Flanders.
So no, I (and I think most of us) wouldn't be offended by it, but the problem are the time periods of the ages in the game.
The golden age for the Flemish (14th/15th C) and that of the Dutch (16th/17th C) both are in the Exploration Age time period right after each other.
But we were then the same people and you really have to look at modern borders to say that was Flemish and that was Dutch.
So you can shift the Netherlands to the modern age and that would be fine (there is more than enough to build a Dutch/Netherlands civ here too), but then you leave so much of our glorious past behind unused.
This is one of the reasons I really dislike the ages system. You have to start and cut them off at a certain timeperiod, where you now get civ choices that can cater as much people as possible (like the Normans).
And even though we get 30 civs at launch, it still feels a lot less then say civ 6 because you only have 10 per age with each a restricted choice of civs.
And even tough a civ flourished in multiple ages they will still be locked in one age for the most part (with an exeption for China and more or less India).