I agree that a Netherlands that includes Belgium would be a strong country. it might not be as strong as the major powers, as Germany, Britain and France, but still stronger than both the Netherlands or Belgium. It would have the coalfields of Belgium, so an earlier industrialization is likely and combined with the trading capabilities of western Netherlands (and Flanders) this Netherlands could become a very wealthy country.
I doubt the Netherlands would be interested in any African colonies. It already has Indonesia, Surinam and a coulple of Carabean Islands. It doesn't need a prestige projects in the African jungle, like the Belgian king got. Ghana is possible, the Netherlands did own various trading posts at the coast, but I believe (I am not certain though) that it wasn't a continuous territory. I believe various British trading posts lied scattered between the Dutch posts (and I believe there where some Danish ones too, although they might have been sold to the British). To form a reasonable colony the British and the Dutch must form some kind of deal. Either one sells its colonies to the other (most likely the Netherlands the the UK) or they trade their colonies so they have a nice continuous colony. I elieve it is more likely that the Netherlands will focus on Indonesia and the areas around it. With twice the population for the Netherlans I could see northern Borneo and eastern new Guinea becoming Dutch and maybe some closeby pacific islands.
The Netherlands and belgium remaining united could lead to some interesting shifts. For example I believe that the language barrier will ly more to the south than is now the case in Belgium. With a large Dutch majority in the country the status of Dutch will gain a lot more prestige, even if a large reason of the Dutch wealth lies in the coalrich and industrial south. I can see Brussels for example remaining a Dutch speaking city and bilingual cities that now lie in French speaking Belgium could end up in Dutch speaking part of the Netherlands. This could lead to an interesting situation in the twenthieth century that the Walloon Dutch would have the same problems as the Flemish Belgians have now, but with the same problem as the Walloon Belgian parts have now: the large unemployment because of the collapse of the heavy industries.
Another interesting thing, at least for people who have some knowledge about Dutch political history, is that the netherlands would be a Catholic majority country. OTL it was basicly 60% protestant and 40% catholic. If you include just Flanders it would become 40% protestant and 60% Catholic. Including Walonia it would have been an even larger majority. This could lead to very interesting situations if pilarization still occurs. I doubt that in this case the Dutch protestant church will split, like it did OTL. If they are a minority I doubt that will happen, they will want to remain together and form a stronger (political) front against the catholics (and socialist and liberals).
I personally see a united netherlands not as a very unlikely event. If the Belgian revolt had happened a bit later, it could have lead to a liberal revolution that would have had support in the northern parts of the netherlands too; leading to a more liberal Netherlands as happened in 1848 with the introduction of the new and more democratic constitution. this would basicly solve all problems the Belgians had with king Willem I. And that is not the only option, without the French involvement the Belgian revolution would have failed. With Prussian or Russion involvement the French could have been beaten. And remember the British didn't care one bit about the Belgians as long as France didn't got it. If the Dutch king and even more so the Dutch crownprince hadn't acted so stupidly the entire revolt could have been prevented.
A united Netherlands getting involved in a alternate world war I. Possible, but unlikely. What would they have to gain? French Flanders? East Frisia? I don't think they cared enough about it. Also i am not to certain they would automatically side with Germany. With a large (and rich and thus influential) Walloon population there would be a lot more sympathy for France. I think that neutrality would have been the preferred option. Although one party entering the Netherlands as happened OTL with Belgium is most certainly possible.
The interesting part is what would have happened to Luxembourg in such a situation. There would be less reason to try to sell it as it was tried in OTL. Luxembourg (including Belgian Luxembourg) is now conected to the rest of the Netherlands. It would still be in personal union with the netherlands, but it would be more intigrated into the Netherlands. What would happen to it during the German unification wars? Would Prussia decide to attack the netherlands and try to annex luxembourg, not unlike Denmark and Holstein? For those who suggest that prussia also might like to include Limburg, please remember that without a Luxembourg split, Limburg would not become part of the German confederation and there exist no good reason to annex it.