Actually, the interesting thing about Belgium was that it was mostly the Brits who wanted it.
https://en.wikipedia.org/wiki/Talleyrand_partition_plan_for_Belgium
This was a proposal send from France to Britain. But yeah thats all part of later history.
Whether or not a formable is added, I think the tag should at least exist.
Its a tag that is missing in the history of EU4 and has a good reason for existing.
https://en.wikipedia.org/wiki/United_Belgian_States
I guess it could become a revolter tag at least. It existed for less than a year. But was profound in creating an identity of Belgians.
The southern netherlands was after Napoleon added to the rest of the netherlands as the United Kingdom of the Netherlands to create a strong buffer state to counter France, and I guess later Germany. But yeah. Britain wanted independant Belgium.
That worked well until Germany invaded Belgium in 1914. So much for the Buffer State.
Yuck, that makes me feel disgusted.