The Netherlands including Belgium (and maybe Luxemburg) would be an economic powerhouse, but it would never be as strong as the big three (or even five). Britain, France, Prussia/Germany, but also Austria-Hungary and Russia (and later the USA) would all be stronger. The Netherlands would probably be richer and stronger than the smaller powers like Spain, Portugal, Sweden, Denmark, Italy, etc. It is a missconseption that Brittain broke up the Netherlands because they feared it. A couple of years earlier they helped form the country. The British supported Belgium independence because they feared that Belgium would end up French and the Belgians already controled the southern part of the nation, so it was the easiest solution. If the Dutch had managed to subdue the revolt (like an earlier 10 day campaign) and the French hadn't gotten involved, the British probably would have supported the Dutch (or wouldn't have get involved at all).
In this united Netherlands Wallonia would become the industrial heart, but the Dutch (and I include the Flemish in that term at the moment) would outnumber the Walloons greatly. The Dutch will not be second rate citizens, like the Flemish were in Belgium, there are just too many of them and too many of them are too rich or influential, but I believe the opposite wouldn't be true either. The Walloons will probably be unproportional influential as a large part of the wealth of the country will come from their lands. A good reason for them to want to remain part of the country. A industialised south could easily kickstart an industrial revolution in the north, which was OTL lacking.
I suspect the Dutch language will do better than it did OTL. Because of the Dutch speaking majority in the country Brussels will remain a lingually Dutch city and so will the surrounding areas. The Franco-Dutch language border will lie to the south. Waterloo for example will still be Dutch speaking.
Keeping the Netherlands together is most certainly possible. If it wasn't for the clumsy reaction of the crownprince the revolt could have been prevented. If the king had been more accepting towards the Belgian grievances, the revolt would never had occurred. Without the French help to the Belgians the Dutch would have beaten them. If the Prussians and Russians had helped the Dutch (and they were planning to, but were distracted), they might even have beaten the French (certainly if the British would have helped them to prevent Belgium falling in French hands). My favorite scenario is a later Belgian revolution which coincides with a Dutch liberal revolution. Both basicly want the same goals, so they will cooperate, maybe even leading to a Dutch republic, which includes Belgium and the Netherlands or they might settle for the constitutional monarchy of OTL.
The natural ally of the Netherlands will no doubt be Prussia, although the situation in Luxemburg might sour that relationship. With Luxemburg connected to the Netherlands they might want to fully incorporate it into the Netherlands. As Britain did not "betray" the Netherlands in this timeline so relation with them will be better than OTL (although they never were that bad, with the exception of the boer wars). The traditional enemy will be France as France would still want to annex the southern parts (than again France is the traditional enemy of a lot of nations).
If we look for new colonies for the Netherlands, I think Ghana (or the Goldcoast) would be the best guess. The Netherlands already had colonial outposts there, but later sold them to Britain, because it didn't consider them useful enough. Maybe now it will kepp it, because with a larger and richer country it can invest more in its colonies. But the most important colony for the Netherlands will remain Indonesia. Colonial expansion will probably happen there. I suspect that northern Borneo might end up in Dutch hands, possibly eastern New Guinea, the Solomons or other islands near Indonesia. I don't think Congo will end up Dutch, as the Netherlands didn't have any intrest there and didn't need a prestige colony. My best guess is that it will end up Portuguese.
BTW I think this belongss in the pre 1900 part of the forum.