Interesting demographic facts.
The United Kingdom of the Netherlands had a population which was split between 2 million Dutch, 2 million Flemish and 2 million Wallon
By 1900 Netherland had 5 million people and Belgium 7 million (the numbers is rounded to nearest million)
In Belgium the Flemish now made up around 4 million and the Wallon 3 million.
This mean that Netherlands grew 150%, Flandern 100% and Wallonia 50%.
We fundamental had a society and development (as Wallonia was the centre of the industrialistion) which favoured the Wallons, and the Flemish population grew much faster. At the same time the Dutch population grew even more. If the Flemish could have a similar birth rate as the Dutch the UKoN anno 1900 would have 13 million people, if we also included Luxembourg and a slightly lower emigration and higher immigration, we could push the population up to 14 million. Spain at the same point had 18 million people, and with the greater Dutch industrialisation and higher GDP, it would likely put both countries in the same weight class international.
Luxembourg are interesting, in OTL their language are seen as either a independent language or a dialect of German. Here it will likely (official) be seen as a dialect of Dutch (a very weird one), at the same time as a integrated part of UKoN it will likely see greater development. At the same time the Wallon Luxembourger are more likely to emigrate to the Wallon industrial areas in Western Wallonia, while Luxembourg instead will see German immigrants. As Wallon Luxembourg was always one of the least developed and most conservative areas of Wallonia, I can see a less proud identification with the liberal and secular Wallon identity. So it may be one of the few areas where the "Dutch" language win terrain. In the rest of Wallonia there will be a too strong Wallon/French identity so Dutch will have a lot of trouble spreading there (so the language border more or less stay where it is, at best I can see all of South Brabant going fully Flemish/Dutch).
The development of cities will also be different. Brussel lies well as the capital for Belgium, in the middle of the country at the language border. But without a Belgium being independent, it really serve little purpose, it do not lie at the major coal fields, not central on a transportation route. Most likely Brussel will be reduced to another Ghent, just another regional centre. Instead Antwerpen will be seen as the great South Netherlandic city, while Liege or Mons will be seen as the capital of the Wallons.
Maybe UKoN will set up/keep a few colonies in Africa, but really don't matter much in the greater perspective, what matters is that there position in Indonesia have been improved, and we may see more competion between Calvinists and Catholic missionaries in Indonesia (causing extra activity), maybe putting Christianity in somewhat stronger position. At the same time the bigger population in the homeland, will likely lead to a bigger Indo population (maybe a increase from 1% of the population of Indonesia to 2-3%).