So if we assume The Netherlands kept Belgium in 1830, will this butterfly the murder of Frans Ferdinand away?
If the answer is yes, what are other causes of ITTL WW1 (which will off course be very different)?
If the PoD is in the first half of the 1800s and you want to figure out how to get a Great War similar to OTL's in around the same year then there's really nothing off the menu in terms of possible catalysts. It depends on how an author of the TL sees to get there, because Europe no longer having an independent Belgium changes the foreign policies of France, Netherlands, Britain, and Prussia/Germany.
A lot is going to be butterflied, especially if the Walloons break out of TTLs Netherlands at some point and the geopolitics surrounding such an event. By the time you get to 1870, Europe may be a totally different continent, and by 1900 you'll have totally different alliances.
One direction is an instigative France enflaming ethnic tensions in the Netherlands with a backdrop of their rivalry with GB that didn't really subside until the end of the 1800s in OTL. It's easy to imagine in this world's WWI for GB and France to be on different sides.
If you are authoring a Timeline, OP, you have a lot of freedom to steer Europe as you see fit or make it congruous to OTL as much as possible. There are a number of ways Germany could resemble OTLs version from the PoD, and a number of ways it could turn out far different. Balkans are gonna Balkan, so that could always be your powderkeg, even if Bulgaria comes out the dominant Slavic state instead of Serbia. So that choice is also yours stemming from major changes in the 1830s. Or you can go vastly different and have the "second coming" of Skanderbeg rise up and unravel the Ottomans from an Albanian focal point. It's up to you. (Muhammad Ali was Albanian after all. I've found that an interesting story to explore).
From my interest in the region it's the effects the resources have surrounding the Rhine and AL. Let's say Germany had Luxemburg from 1871 to the time the Great War broke out, they'd be moderately more industrialized with a massive flex of iron. Now say Germany had the iron regions in Longwy Briey as well, same thing but now France just lost their trove of iron and as a result would be less prepared for a Great War.
I bring this up because if the Netherlands had all that Belgian coal and iron, possibly even the luxembourgish deposits, they'll have a stronger economy by the time the Great War breaks out. Definitely a much stronger navy and a much better chance at securing their overseas possessions if they get involved in global warfare.
Having written all this, I'm wondering if you, OP, have any specific questions, considering the opening post is rather vague.
